GD&T : How to measure Flatness

If tolerance on drawing , limit 15
It can use , Because this work piece is limit +6 (max of oversize), -7(least of lowersize) = 6 + 7 = 13 .

But . . .
If to tolerance on drawing , limit 10
It can't use .

***Measure dot to dot at least 5 points + up to surface .