I think in the case of your crossed canoe block for the pieces that will end up being an outside edge it won't be too difficult. For basis of discussion I am looking at the upper left hand quadrant of the block. The two pieces that will be on your outside edge in that quadrant are the large white right angle triangles. Measure the distance from the top left hand corner over to the right where it switches color. Add your seam allowance and then another 1/2". I suspect the measurement will be 7" or very close to it in a 12" finished block. Now measure down to where it intersects with the "canoe". again, add your seam allowances and 1/2". I bet that measurement will come darn close to being 5". If you cut a rectangle to this measurement, on the grain, I bet you can make one diagonal cut from upper left corner to lower right corner and the resulting peices of fabric will be just right for PP the two white right angles in that quadrant of the block and also leave you outside edges on the grain, with just a bit left over to trim away.