After you get it floor tillered,and long string tillered,in other words when you get it to brace[by the way I try and do that as soon as possiable] then I put it on the tillering board,I use the rope/ pulley system like many others on here. When you start to pull the bow,decide what draw you need and what weight you want,never go over that weight when pulling the bow,the first time you pull it may be something like 50@15 inches,you check out any problem with the bend and scrap there,you just keep adjusting the tiller while taking off wood,for example the next time it will be 50 at 16 inches.Just keep working with it as you move out on the draw length till you get the draw you want at the weight you decided.I usually go a few pounds over the weight I want to allow for shooting it in and final sanding. To answer your question,yes you start with more wood than you will wind up with in the end. I am always careful with set diminution's,a lot depends on the wood type
and density.So all wood is different.50 lb was just an example,you may want 40 or 60 it don't matter just don't go over that when you are tillering. That will help from damaging the wood and set.
Hope this helps. You might want to go to Jawges site,it has a lot on this.
Pappy