You multiply the FINISHED size of the block by 1.414 to get the FINISHED width of the block when on point.
An 8" FINISHED block on point will be ~11.3" across (the measurement of the FINISHED diagonal).
If you have created a design where you want six on-point blocks/squares that are ~10" across (because you want a 60" wide top), you would reverse the formula:
10" divided by 1.414 = 7.07" (and round up or down),
THEN add the seam allowance to cut the squares.
For blocks, I'd want them to be the 7" (or 7.125" or 7.25", depending on how you round) FINISHED.