Yeah like knoll said if you trap it trap the back, that will drop the weight as well. I don't think the timber hitch is slipping if it is tied right, but as you scrape wood the brace hight will get lower. Try for 20lbs at 20" that should set you up for 45# figuing 3lbs an inch. Now like howatd said about the tree, once the tiller is good at 20" what i do is start shooting it at short draw, say 25 arrows at 20" then i just gradually work it out to full draw, have someone take a pic about 24" with an arrow nocked and put a pc of tape on the string so you nock it in the correct spot , say the thickness of the arrow aboce center, you got this about wrapped up, round up the handle some before you shoot it so you dont shoot the feathers off