At some point, thinning and reprofiling starts getting close to 'knife making'... The best advice I can give you, is that you need to start by imagining the knife you want. As others have shown, try to draw it on the blade, and work roughly to that profile. Realize, that the profile you draw, still isn't going to be what you want, and you'll have to tweak it after to get it right. Give yourself room for this. Treat a big reprofile essentially like you would, if you were making your first custom knife. Imagine the blade you want, and give yourself a little wiggle-room to tune it in. Wanting a flat, tends to result in too flat of a blade. Creating a belly tends, to be too bellied. Give yourself some room for easing these extreme shapes. Your idea of a good blade profile probably isn't right. Your hands will struggle to create the shape as smoothly as your mind imagines. Those shapes in your head, will not be as good as they need to be, because really tiny changes in profile effect the physics of cutting in a big way. Start with an idea, and refine it. It's the only way, until you've done it countless times.
I'm not talking about minor stuff...
I've got a Nakiri right now with a tiny frown on the back-flat that needs fixing, and a tip that needs the radius extended up a bit. I'll bread knife the frown out at the back half of the blade, and bread-knife the whole blade with a little rocking motion to maintain the profile so it doesn't get too flat. Then I'll bring the secondary bevel up higher around the radius of the tip to extend it, at roughly a 10-dps angle, until I've created a 50/50 apex at the tip. At this point, the blade will look like a mess. It'll be variably thick BTE after this treatment, but that'll be fixed in the next step. I'll establish a consistent mid-point thinning bevel (Establishing that consistent mid-point bevel will be my guide for everything else.), then lower it to zero the whole edge, then raise it to get my bevel height where I want it, then rock between the two to even everything out on a mid-grit stone. My off-hand primary bevel will be flatter than my dominant primary bevel. After this, I'll put on a micro-bevel in a few strokes, that will be consistent, because everything else is inline at this point. Getting the edge profiled so it cuts good, then establishing that mid-point primary thinning bevel, is critical.
That's minor work. Some jobs, like
@Dominick Maone showed above, are proper reprofiles.
Establishing the profile is your critical first step. Give yourself some wiggle room to get it right. I still believe in the bread-knife method, unless you have really minor edge inconsistencies, where I'd just say that you should follow the profile, and sharpen them out normally at the secondary-bevel level. Then create facets to thin the blade back down, and make sure each facet is of consistent height, even if it means working some parts more than the other. Once you have one consistent facet, the others will fall into line. Then blend the facets. Then do the aesthetic/finish sharpening.
Don't make your initial primary-grind facet too low, or you'll distort the profile. Don't make it too high, or you'll raise the grind way too much. Create a consistent mid-point facet, on pre-ground blades.
This is also an opportunity to change your grind... Look at the spine. If your spine is distally tapered, and you keep the bevel height consistent, the front of the knife will have a taller, less convexed grind than the back portion, even though it aesthetically looks the same height. To keep a distally tapered blade performing consistently along its entire length, you have to lower the grind at the front, and raise it at the heel, as per
@Kippington. Conversely, you can taper the grind so it's thinner and more laser-y at the front, and more robust at the heel, on knives with no distal taper, by having a low primary at the heel, and progressively raising it towards the tip, which is more in the style of how Jurgen Schanz (Of Schanz Messer.) regrinds such knives.
Would I breadknife a single bevel? Unless it was a very minor profile issue on something like an Usuba, probably not. I'd approach it at a primary-grind level, and keep going at a 45-90 degree angle with the shinogi, until things were straightened out. This methodology preserves the Ura and Shinogi the best.
Remember, when you're thinning, going with and along the primary grind, makes it flatter over a larger area. Going across the grind, at a 90-degree angle, convexes it more, and puts more force on a smaller area. Angles in between are some hybrid between those two extremes... Creating a flat bevel, and a convex bevel. You're using your own innacuracy as a human, to your advantage. You can shift the flattness/convexity, and size of your grind area, by changing the angle of approach.
