One nice thing about feeding in with the cross-slide is that you can use the DRO to check DOC. Or, you can set the cross-slide to 90 and then use the dial with no trig conversion. I don't care about that, because I do it this way:
1) Cut the relief
2) Touch the threading tool off the OD, and zero the DRO/dial on cross-slide and compound
3) Plunge (into the air of the relief, no metal contact) until you're at the minimum DOC for that thread.
4) Lock the cross-slide
5) Dial the compound back out to clear the part - when you thread, just dial it back in until it hits "0". That should have you perfect or slightly oversized.
6)Measure over wires and finish to dimension with either compound or cross-slide.
I prefer to run the thread at ~29deg with the compound, and then take the final fine cut to dimension with the cross-slide. I find this gives the best finish using my tooling. But depending on the setup, I might thread away from or towards the headstock, with an upside down or right side up tool, or even a boring bar on the backside of the part with a threading insert.
I also prefer to do the math and use a single pitch wire when using a partial-profile tool. This only works if you know your OD precisely, because the mic rests on the thread crests on the other side of the part. I take the extra time to hit my OD to within a couple tenths, and then DOC and a single-wire let me nail the PD.
But, I normally use a full-profile insert, and as there are really only 5-6 common thread pitches, I have the numbers worked out in advance for all of them. I cut a few thou oversized, cut at 29deg to minimum thread depth (per steps above, but finishing a little oversized because the OD was oversized), then plunge straight in until I hit the OD I want (usually 1 thou under max). Then I check the PD, but it's always perfect with good (ISCAR) inserts.
The only time I cut threads to fit (instead of to spec) is when I'm mating to something out of spec. Some 'smiths like to cut to a really close fit, but it doesn't really buy you anything. Thread form and alignment is the key, it'll self-center and align off the shoulder.
That brings me to another point - the shoulder is where the magic really happens. Take the time to cut a really perfect shoulder, with appropriate relief or radius at the inside corner. Be sure you're not getting a little play in the carriage or compound when you cut it - I prefer to set a hard stop and keep a little pressure on the carriage handwheel while I face the shoulder off.
There are a huge number of ways to skin this cat. Find a set of techniques and tools that work for you, and stick with them.
