It is size memory for me. The tank will be the same size for a given distance. I let my brain decide for me. It takes lots of shooting to get the range down.
Quoted for truth. I know range more by size of target than position on the reticle. It has taken a lot of shells down range to get there. Once you have acquired this ability, you will have nights that you can just point and shoot at varying distances and nail vehicle after vehicle.
Get a buddy in the TA. Have him move away a roughly set distance and plug away at him. Have him do it with different tank models too as they all have their "size" differences at different ranges. Start stationary. Then work traversing shots where he is moving, then shots while you are both rolling. It will happen for you. Practice, practice, practice. Best of luck!