It depends on what size block you want to end up with. If you want a 12" finished block, a 15" piece is long enough for the longest log and could be subcut for shorter lengths. You might want to check the length needed for 2 logs that form the next to outside log of the block to see if the 15" is enough to make two of them. A lot depends on the finished size of the piece and if you want to have enough that some of the blocks can have the same fabric in the same log. Best of luck to you ding this. I always overthink things so this may not make any sense. In that case, please disregard as the ramblings resulting from a senior moment.
Joyce in DE