Tillering is finished when:
The limbs bend evenly throughout aswell as symmetrically
The string is long enough that the fistmele/brace height is ~6 inches and you have the desired draw weight at your draw length.
I've yet to make a >35 lbs bow despite trying so don't despair if you can't make a 50lbs bow immediately.
Pic related is another bow, this one I made for a kid. Made of yew. As you can see I didn't reflex the limbs enough and after taking set they are curved even when unstrung. The backing is again rawhide, this time oiled as an experiment. It's fuzzy and ugly, don't oil rawhide. You can rasp and file rawhide btw.
At the very least, skim Bowyer Bible vol 1:
http://www.mediafire.com/file/26sdzs1ql4viylc/The+Traditional+Bowyer%27s+Bible%2C+Vol.+1.pdfhttp://www.mediafire.com/file/hzh6mb4c1ciykte/The+Traditional+Bowyer%27s+Bible%2C+Vol.+2.pdfAlso, probably screencap these posts lest you forget.
Good luck.
