Making up your own pattern can be hard. When doing your own, I would not do any triangles. If you have everything in squares or rectangles, it actually can be pretty easy to make a pattern.
What you need to do is first make a pattern that DOES NOT include your seam allowances. This will ensure that the final pieces will all match up. Then once you have the pattern without seam allowances, then you add ½ inch to the length and the width of each block and that becomes your seam allowance.
If you are starting with a piece of fabric that is a set size (like your panel), you’ll want to subtract the ½ inch from each side first, then do your pattern without the seam allowances and add them back in after.
So for instance I want to make a quilt that is 10 blocks across and 10 blocks down. I want the quilt to finish at 60x60. That means each finished block has to be 6x6, I add my ½ inch to the length and the width of each block and I have blocks that are 6.5x6.5 …
Another example …. I have a 10.5x10.5 piece of fabric and I want to surround it with squares. Since 10.5x10.5 is the actual size I need to subtract ½ inch to get the finished size, so I have a 10x10 finished size. I can surround that with 12, 5” squares (2 on all sides and 4 in each corner) evenly. So now I need to add my seam allowances … So to make this quilt I need my 10.5x10.5 piece and 12, 5.5 pieces ….
So it was very difficult to make something using a 15x15 panel because the finished size is actually 14.5x14.5 (unless the panel was actually 15.5x15.5, which would be much easier).
I hope some of what I said makes sense and helps you figure out your next quilt!!
Cheers!
Rachel