Well, one way would be to load some up on a ladder test to a super tight group in your riffle, get the velocity of the best group, then you would input your basic info (zero info, velocity, bullet length... anything you have), then shoot some good, consistent groups out at about 700 yards, then input the amount of elevation needed to get those hits into your ballistic app to "true the BC"
Before I had ballistic calculators, I just loaded a shit ton of bullets and got solid "yard line" DOPE at 100, 200, 300, 400, etc.
That was literally before I knew anything about BC's and was doing nothing but loading exactly one 308 load with 175 SMK's, Lapua brass, Rem 9 1/2 primers... I lucked out and threw together 10 rounds of 40.5g of IMR 4064 and those groups were .75 MOA at 100. I didn't even have a chrono at the time. I just made a bunch of that load and shot the shit out of it. 6 months later I was able to chrono the load at about 2700 with SD ~10 or so. All with my fancy schmancy Lee reloading equipment.
It's a lot of work, but you'll get a bunch of trigger time and no-shit info.