You cut D9P twice - if you made a nine patch from 5" squares, your unfinished block would measure 14". You cut this again into quarters, yielding 4 blocks of 7" unfinished or 6 1/2" finished. I think of these as the final blocks and do my layout and join my rows using these quarter blocks but if you sewed four of them back together, you'd get a 13" finished block. If your original patches were 4 3/4", you'd get a 12 3/4" unfinished block by the above process. I'd do this then trim down, otherwise you'll be fiddling around with 1/8" :lol:
The formula is:
(size of patch x 3) subtract 1" ( 4 x 1/4" seams) = unfinished size of nine patch, then subtract 1/2" (represents 2 x 1/4" seams) = unfinished size of new block if nine patch is cut into quarters and sewn back together.
Reverse the formula to work back from your unfinished final block:
Unfinished final block + 1/2" + 1" divided by 3 = patch size so:
12.5" + 1/2" = 13" + 1" = 14" divided by 3 = 4.63", which is 4 5/8". :-D
NB - I made a mistake (see post below, this formula has been edited and is now CORRECT! - I hope! :lol: )