You could 'zero' at 100 yards, check the drop at 200, then go to a ballistics calculator, set up for 300 yards, zero at 100, use the same weight and model bullet you are using, then, fiddle with the muzzle velocity number until you get the SAME drop from the calculator at 200 yards as you are getting for real. That will give you a very close idea what your muzzle velocity is. Take that to your reloading chart and verify powder load. NOTE: make sure your barrel length matches the load data chart. You can lose up to 40 FPS for every inch below loading chart barrel length data.
I have a 7X57 that shoots REAL low(10 inches), but then, I'm underloaded from minimum by 3 grains. Long story, chasing a problem. Got it identified and fixed. Now have to restart the load process from the minimum.
Yes, humidity and temperature must be taken into consideration, as well as altitude. (added from follow on question)