I think what you're supposed to do with the BDC is zero at 100, then shoot 200 using the 200 crosshairs and just see where it's shooting. I doubt it would be right on the crosshairs even on flat land. Scope companies have a good idea with the BDC's, but just can't make them to exactly shoot for all. Even if you had one for 7mm, it would be off depending on the bullet, powder, cartridge, etc. you were shooting.
You juts have to move out, establish a pattern and remember where it shoots at 200, 300, and beyond.