i have used the pathagorean theorem as well and it always worked pretty close for me but the best piece of advice that you can get in my opinion is to know if you are shooting at and angle down with a bow you will tend to shoot a bit higher at 20 yds than on the ground at twenty but not but much . Same for shooting up hill, you will shoot lower than normal due to the angle and by a little as well. just aim just a slight bit lower or higher and you should hit right on the mark but a couple of inches isnt goin to make two much difference unless you are shooting a farther yard in which the inches increase a little. But a2+b2=c2 will usually work quite well if worked out correctly. just dont forget to take the square root of c at the end. 16 yrds is 48 ft, squared is 2304, 15 feet up in the tree squared is 225, so 2305+225 = 2529 and square root it is 16.7 which only elongates you by .7 yrds and at that theres gonne be virtually no difference in shooting. just the farther distances that will make any difference. prolly no real affect until you go to about 40 yrds. Just put that problem on there just so you can see if maybe you done something wrong, not that you did or anything but it has always worked for me.