use the stuff around the person, judge by size. should only take 1 - 2 shots, 3 shots max. otherwise you give away your position and you gotta change before someone comes to get you and knife you in the back.
You can also just go by memory, I start to see how far by how big the person appears, at distance when they start to become hard to see its a bit harder. You can also watch the bullets trajectory and make ajustments accordingly.
just as a note, the M98B is more accurate and has less bullet drop over distance. In non hardcore, at distance, the MK11 sometimes takes 2 headshots to kill, most likely if you're using the MK11 or SVD for long distance then you're just gonna be plinkin for the body. I prefer to use semi autos for closer maps like OM or on rush if we're pushing up quick then I'll get the MK11 and chuck an acog and supressor on it.
