I think that halving the size to simulate distance is not a good substitute. From my own experience, I've been getting much more consistent from 10-13 m just out of necessity (limited backyard space). But when I get to go to 20m, a target 2 times as big is much more difficult to hit than the smaller target at 10 m.  As others have said, wind resistance and the accompanying extra drop in trajectory, coupled with any curve of the projectile from spin and the extra power that is needed to throw, the increased difficulty is more than the sum of the parts.