Well, I'm way late to this game, but I have a perspective that hasn't yet been mentioned in this thread. It's one that has been talked about elsewhere. Given its nature, I'm surprised no one has mentioned it.
The method of mounting will have an impact on flatness. You can measure a block on the bench til your eyes are all squinty and possibly know nothing of how it will look as-mounted.
Many blocks are mounted with screws to holes in the motherboard. This means you've got the die pushing up in the middle and four constraints at the corners. As a rough ballpark, I tossed some numbers into one of Rourk's equations. It isn't perfect, but close enough for roadwork.
Say you have a block ~70mm on a side with circular die (yeah, I said circular, the equation is shorter) of radius = 5 mm. If the (copper with E~15E6 psi) base plate is 5mm thick and responsible for carrying the load, you can calculate deflection vs load. At 20 lbf, this comes out to ~8E-5 inches. For the metrically challenged, that's about 2E-6 meters.
The situation is obviously different if you have a mount that presses down directly above the die.
If nothing else, at least consider "flatness" below this level to be of extremely limited benefit.
I ran this through in a hurry, so no guarantees to accuracy. . .
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