Just make sure your speed is stable before you callibrate. The Boston accelerates much faster than the B24 or B26 so your sights will be knocked off true very quickly unless your speed, height and direction are constant.
As for sticking with the A20 - it's a different package, for different tasks. If you like the Boston, use it. I imagine you like it because it's quick and you can do medium to high level bombing in it, which you can't in a Havoc. The small bombload is the draw-back but with three of them you'll do more damage than an A20. I find it easier to shoot down one Havoc than to shoot down three Bostons. Usually;) The A20 certainly packs a heavier punch.
If you don't mind taking a light bombload, then the Ki67 might be your thing. It's fast, has a bombsight and it is well defended. I took out a 262 last night flying the 67. The 262 made the mistake of breaking over my Ki67s and got three mid-upper turret cannons as a New Year present. Three Bostons would have died very quickly. Anyway, sorry I'm getting off topic...
KD
