A recent set of posts, I think by Baldy and Glen, suggested half the tang width for the wedge as a starting point. I'd never made that connection, so I usually estimate based on blade width and grind - a wedgey blade needs a wider wedge if you want it to sit halfway or deeper, and a full hollow needs a narrow wedge unless you want the blade to sit deep. Personally, I don't like my full hollows to sit really deep (because I prefer narrower scales like vintage razors), so I often start my full hollows with a 1/16 piece of stock. My heavy grind blades usually start with 1/8 stock. But there is variation based on scale shape and such.