I hate to repeat the already stated, but he is right. My best advice is to practice your butt off and with a resonable amount of shooting at various distances you will start to see where you are capable of making "solid" shots and you will also start to see at what distance your "chancey" shots start to develop. The two most important words I can give you are Consistency in your practice and Confidence when taking a shot. If neither of these exist you will never feel comfortable at making ANY shot whether it be near or far.

Personally, I try my best to get the desired animal within 15yds just because I am not as experienced (been doing this for 2yrs) as others may be. BUT, if everything felt good, I may be tempted to stretch it to 20yds, that is my personal limit. Like I said, go practice from every possible positin and distance imaginable and you will start to see your limitations become clear.