My guess is if you have problems identifying 30 yards while practice shooting you are going to have tons of problems with a big buck in front of you. Sounds like you need to work on your 'range finding' more then your shooting.

I bought a rangefinder for about $150 (bushnell), works great and has really helped me estimate distances while practicing. i just walk to where i think is 30 and then range it to be sure. i'm usually off my no more than a couple yards now.

besides, distances look different 20 feet up in a tree.