When it comes to estimating distance & what shooting position I use, I have 5 distance brackets which generally work fairly well.
1) Short way.................................................................................0 to 120 or so. Off hand or, a handy rest.
2) Little bit long way.................................................................120 to 300. Steady off hand rest, car bonnet etc.
3)Long way................................................................................... 300 to 500. Car bonnet, truck rack, fence strainer post, prone.
4) Fucking long way................................................................... 500 to 800. Good shooting bench, prone, lay on car bonnet etc.
5) Can hardly see the cunt...................................................... 800 to 1200. Prone on ground, lay on car bonnet or truck back.