I do it the same way Jimpic does it, I do it in increments but I use foot steps and one footstep equals that of one yard, so I ask my self, How many steps from the target to an object (rock,bush,ect.) between me and the target and then how many steps from this object to me?
I am usually pretty good at judging distance and its usually a little forgiving up to 30-35 yrds, but if your 5 yrds off beyond this range, it makes a big difference. It will all come to you in time and the more you do it the better you'll get, good luck, Bobby