In peening the tang, the slot needs to be fitted to just slide over the tang without more than light finger pressure to seat it. The up edges of the slot are slightly beveled. The tang must be trimmed to leave just enough to fill the slot bevels, plus a tiny tad more to over ride the slot edges. The tang in that region must be soft enough to peen well. To prevent cracking as you peen, the edges and corners of the tang are beveled and or rounded. It is a sharp 90° edge that usually cracks. Try to peen down evenly all around and in the center, then dress off the excess height. Many light to moderate hammer blows do better than just a few hard blows.
I run a fairly flat bevel from the edge, up about 1/3 to 1/2 the blade width, then blend almost flat, but slightly apple seed. A straight edge set in the middle will show about .010 to .015 of light on each end depending on the blade width and thickness, and what the blade will be used for. Some even less, and some more. I leave a flat parallel area near the guard, or near where flat slabs need to be fitted, but try to blend the flat where it is not easily noticed. This flat is very handy to true up a blade in a drill press vise, in order to drill handle slabs at a reasonable 90°. It also allows easier tight fitting of a guard. When drilling for pins in a slab with an irregular surface, I super glue it in place on the tang with a few tiny drops, lock in the vise, use wood wedges to keep it from dropping down, then drill my holes using the holes already in the tang as a guide. A little heat, or just a smack with a wood block will knock the slab loose.