hey shiny I used to use the same calculator that ray posted a link to eventually I found this one
http://www.csgnetwork.com/righttricalc.html I find it a lil easier (for me any way every one is different) this one I use now is a "right" triangle calculator. so the one ray posted is if your thinking of the whole blade for me I find it easier to think of one side at a time and I think the "right" triangle calculator a lil easier for that. I also try to find the angle for how high I want the line to go up and not the other way around if that makes sense for example if you look at that link....say the stock is .187 thick but you want to leave a lil thickness at the edge so lets say .187 - .02 =.167 now if you think of one side at a time divide .167 by 2 = .083 so .083 is "SIDE B" on that calculator and now say you want the grind line to come up .5 so .5 is "SIDE A" so if you plug those to in and hit "calculate" it will show "ANGLE 3" is = to 9.43 so that is the angle you want to grind one side on so if each side is done at 9.43 the total 18.86 I may be explaining it very complicated but for me it makes it easier thinking one side at a time .....also you can figure out side b if you plug in side a and angle 3 as long as you have 2 of the sides or angles you can calculate the rest... try it both ways and see what works for you and of course in reality you cant grind something at exactly 9.43 of a angle but then you got something to shoot for...when I do a full flat I do it complete free hand if I do bevels half way up I have been using the bubble jig lately its not like a normal jig very close to freehand its more of a guide than a jig to me. I can do it freehand but it makes it quicker to get the sides even with the bubble jig...try differnent ways and see what works I used the one ray put up but thinking of it as each side different using the other calculator ended up being easier for me different strokes different folks