The thickness of a Fiddleback handle is not dependant on the steel thickness or tang tapering. It actually depends more on me and my performance during shaping of that handle. Each one is different, and I stop when I see the curves relating to each other in the proper way. That is complicated to envision. Imagine me sitting next to the grinder in an office chair with my mask and earmuffs on. The 8" wheel attachment is running as fast as I can get it. I shape handles with a brand new 36 grit blaze belt, so there is a lot of cutting power going on and the work is basically getting done a foot from my face as I sit and face the side of the wheel. This is the position I am in when I start to shape the handles. At this point there is no set thickness to the handle, but I have a general idea of what I want the piece to look like after this step. I cut 4 hollows in a handle with a palm swell. The hollow should leave more meat on the spine side than the leading side. This gives the handle the proper egg shape profile from spine to edge. If the cut goes perfect the first time, I stop and set all the other curves by that one. If they all go right and things relate correctly then I don't have to ajust the first cut. Every time I have to ajust the lowest cut to meet some new low I have made on the other cuts I have to re-shape all the cuts except the lowest one. Think of how your barber cuts your hair. They make a cut, then cut everything else to match that cut. If they cut off too much somewhere else, they have to cut everything down to match the new low. All stock removal works this way. When everything matches up and looks like a comfortable professional knife handle, I stop.
Many knives don't have to deal with this because they just round the edges of the scales. That way both sides always match. This makes for a much more consistantly shaped handle. It would also mean that a tapered tang knife would be thinner at the pommell. Thats not the case for Fiddlebacks. I basically ignore the tang when shaping. I view the handle as a single entity to shape. My handles are organic instead. It is very time consuming (taking 2/5 of my time each week).
Does that help?
I've done some facebook live video's showing this process if you are interested.