Tagging onto what
@Dthomas3523 is saying, there's a growing body of evidence that velocity "nodes" don't really exist, in that they can't be repeatedly demonstrated at a given charge weight, or across a given set of identical charge weight ladders. So, there's a growing group of people who are starting to abandon that approach, and either just pick a speed to target, such as reloading manual maximum, or look at mid-range performance on target to determine an OCW. Many of the top long-range shooters out there do currently subscribe to the theory of positive/negative compensation, which is another reason to look for your OCW on target rather than across a chrono, since barrel harmonics might effectively "cancel out" variations in MV across a limited MV range.
I'm actually working up a load right now, and I'm trying out Scott Satterlee's approach, which is to rough in a seating depth first, using a "safe" charge weight (you can use manual minimum, or manual "middle" if you like), then run a powder ladder after. Specifically, I did 0.010" increments of increasing jump, starting with 0.040" and going up to 0.100", looking at group size and impact location. This was just at 217 yards since that's the max my local range can offer, and the reason I jumped so far (as opposed to "kissing" the lands up to say 0.020" jump) is some recent tinkering that Satterlee and Mark Gordon of Short Action Customs, among others, which suggests that longer jumps offer a more stable seating depth (i.e., resistant to accuracy issues from erosion of the lands causing your depth to get "out of tune") and longer barrel life. I'm frankly quite fed up with tinkering with load dev, and am hoping this new barrel will dial in quick using this prototype method, and that I won't have to screw with it halfway through the barrel life.
Anyway, so after I checked seating depth and decided on 0.100" (much better group than any other, but similar average POI as 0.070-0.090", I ran a quick powder ladder to look for pressure signs, just a single round at 0.4gr increments (6.5CM, so those are ~1% increments). Never saw sticky bolt lift, but the amount of powder compression was getting pretty up there towards the top of my ladder and I started seeing light ejector marks around 2750-2800 fps so that's my cap (happens to correspond to Hornady's max speed, I'm shooting their 140gr HPBT because it's cheap, I don't need top-flight accuracy and I could actually find the damned things during the pandemic). Now I've loaded up a ladder in 0.2gr increments up to the charge that showed ~2800 fps, and I'm gonna take 'em out to 600 yds to observe group size and average POI elevation ("DOPE") to feed my ballistic solver an average MV (this is Frank's method, makes a lotta sense to me after chasing velocity nodes). My biggest concern with this next step is my own ability to shoot good groups at 600yds haha, but we'll see how it goes.
One thing I've learned multiple times over, though, is to always load up a few spare rounds before I head to the range. If I'm doing a ladder, it's spares of the lowest charge weight (in case I get surprise early pressure signs); with seating depth, it honestly doesn't matter so I pick either the least or the most jump. These are sighters, and I need them because I move my scope between rifles regularly, and I hate using up what should've been actionable data just getting on the freaking paper. I also draw up charts with columns and rows for all the info I care about in my reloading book beforehand, because it saves me tons of range time and I'm also less likely to forget a critical parameter if I do it in advance. For example, for my 600yd day coming up, I'm tracking the following:
- Environmentals
- Wind direction and speed (average is fine)
- Dialed elevation for each charge weight group (since I expect to have to dial down as charge increases)
- Average POI elevation (i.e., dialed elevation plus target center offset) for each charge weight group
- Vertical spread for each charge weight group
- Horizontal spread for each charge weight group
- True DOPE for each charge weight group
- Calculated average MV for each charge weight group (back-calculated using the ballistic solver)
- A sketch of each charge weight group (shape matters, even distribution is best and I don't have one of those target photo apps like Ballistic-X since my phone is too old, but if it wasn't I would absolutely get that program)
- Notes for each charge weight group
I also loaded up five sighters of the lowest charge weight, in addition to the five rounds I'll use for the "score" group.
I agree with your plan to break the barrel in with virgin brass; Satterlee has noted that his barrel didn't speed up when he ran tons of jump, but that's just one barrel and anyway pretty much everyone agrees that once-fired brass is better to work with than virgin, plus you can get comfy with the rifle before you start shooting groups that you'll make decisions from. I wouldn't worry about SD/ES, honestly. There aren't a ton of things you can do to reduce them, you've likely already decided which ones you'll do and which you won't (such as annealing, sizing with a bushing or honed FL die, choosing top-tier brass and bullets, and choosing a temp-stable powder), they have a lot less effect on your ability to hit steel at longer ranges than most people realize, and also you'll lose all your hair if you stress about chasing single-digit SDs. Powder increments of 0.1gr seems pretty small, and depending on your scale, that might be better than it can consistently measure anyway; on any mid-sized case, including your 6GT, I would personally (others might disagree) say that 0.2gr increments is as fine as it's worth going, since your ES at any given charge is likely bigger than the average change caused by a 0.1gr powder change. I like the rest of your plan.
There are tons of ways to skin the cat, and I know I've typed out a novel here. If I were to give any advice at all (now that I've given a ton), it would be this: decide what accuracy level you are after before you start, don't pursue nitpicky crap beyond what it takes to achieve your goal, look for ways to simplify your process unless you want reloading to be your only hobby, and always take a few extra sighters with you to the range haha.
Finally, PRB has written some really great articles lately that run directly counter to some very popular conventions in the shooting world, specifically the jump testing stuff and the "Does It Really Matter?" series. Here are links to those, if you can at least believe what the DIRM series is telling you (tiny ES, tiny groups, and hot-rodding a load don't help very much) then that alone will save you a ton of headache.
Hope this helps, and ask any more questions that strike you!