Originally Posted by qbquilts
You need to add in the seam allowance before doing the math. So the 8" finished block becomes 8.5". Plugged into the formula, you get 12.02. Just to demonstrate that you can use the formula OR the "Multiplying by 1.414" trick, multiplying gives you 12.019. Round UP to the nearest 1/8 for ease in cutting, so sashing strips would need to be cut at 12 1/8.
No, the formula is used on FINISHED sizes/measurements.
The seam allowances are either added to or subtracted from the sizes, after or before the calculation, depending on what you're trying to figure out.
If I've got an 8" square block and I want to know how wide it will be on point, it's 8" x 1.414 = 11.3" (then round up).
The unfinished size will be ~12", but that has nothing to do with the calculation.
Let's say I've got a design where I want six on-point blocks/squares that are ~10" across (because I want a 60" wide top).
I take 10" divided by 1.414 = 7.07" (and round up or down).
I would THEN add the seam allowance to cut my squares.
For blocks, I'd want them to be the 7" (or 7.125" or 7.25", depending on your round) FINISHED.
The seam allowance is always a constant .5".
You wouldn't want to perform the calculation by including it.
Actually, you'd pretty much exclude it for any calculation (not just this example).