A 12" square on diagonal is essentially 17". When we do rough calculations you can use 1.5 times the finished size but that's a bit large -- it's how I figure the setting triangles though to make sure they are big enough, a common error is taking the size of block and cutting in half but that's not enough for the seam allowance.
So 4 blocks across with no sashing and no borders would be 68" (4*17). I'd probably go 3 across and 4 down -- but remember when you set on point you have alternating length rows plus the setting triangles. So, if I'm counting it right that would be 3 rows of 4 (12) plus 2 rows of 3 (6) for a total of 18 blocks and add the setting triangles to that.
I've included a layout I drew up earlier this year and now is in my handy notebook, sometimes it helps to actually count.
Also a couple handy sites,
All sorts of calculators and help here:
https://mywebquilter.com/
Simple diagonal site for when you don't want to just do it on a calculator...
https://www.omnicalculator.com/math/square-diagonal
You can have the side measurement or use it to figure how to square the diagonal, depends on what you know.