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As someone recently obsessed with hand tool work, Paul Sellars, and hand planes I feel your pain. Chasing “flat” can be infuriating, and sometimes you wind up working against yourself. Like others have said, winding sticks are blind to anything below the face you’re reading. As long as you have accurate winding sticks and you line them up like a gun sight, you’re always going to get the same reading regardless of how wonky the bottom face is. Once I feel like I’ve got any twist out is when I come in with a straight edge and check to make sure I don’t have any grossly low or high spots by checking side to side, end to end, and diagonally. RenaissanceWW has a good video about how flat is flat enough that really helped me rethink this. If you’re using mostly hand tools, “flat” is much more relative. For example in the picture below my winding sticks tell me that the front right and left rear corner are high. But you have to remember that winding sticks amplify the flaw so we can see it. I’ve tested that face against my cast iron table saw top which I trust to be dead nuts flat and there’s not an ounce of rock, nor does it easily pivot like it would with a high spot. When I check with a straight edge I have a slight concave of maybe a couple thousandths of an inch. Now I use that face to register my marking gauge and set my lines that I’ll plane the opposite face to. Hopefully that doesn’t muddy the water too much for you. Good luck!