RE: distance
At only 12',shot placement won't changeMUCH either.
Shoot the distance from the base of the tree,that will be so close to the actuall yardage that you won't be able to see a difference.
They key is to bend at the waist.If you don't and just point the bow down hill,your manchor will change and if you do that,you will definately need to be practicing from a tree.There again,at 12',it won't change much.