If this question is about your Disappearing 9 Patch.....
I would do as Sharon does:
"I square up the 4 pieces to make sure they're all the same size then sew them together. I'm in the process of making one, too."
This method will allow you to square smaller sized blocks.

After they are resewn together, I would resquare the larger block, by making a template the correct size (13.5") out of something light weight if you do not have template plastic. I would mark the center, diagonal lines and any other lines you may find helpful if you cannot see thru your template. Use a ruler to cut - not the template.
Visually, the lines on the cutting mat are a guide, but I never use them for cutting-just me.

For larger blocks I line the centers up on lines on my mat then use a smaller ruler to trim...for instance if a block is 12-1/2" I line up the centers along lines on my mat then use a ruler to trim 6-1/4" on either side of the center line.

Donna

I don't use the mat lines for cutting either, Marsha.

If the size of the block is larger than the 12 1/2 inch square ruler I have, then I do as Donna does. In fact, I do it that way most of the time which makes sure the center is really the center of the block. It makes it easier for the quilters to quilt when it's as exact as we can make it.

SharonG/FL

I have some see through heavy plastic that I purchased last year. I can use a black marking pen to cut it to size and be able to see this though my ruler. Thanks for the tips.

Wish I had some plexi glass but this will work and I will lay my large 24" ruler on top of them when ready to trim.

My blocks looked fairly square so went ahead and sewed the four blocks together. So need to square the 13 1/2" It will work with your suggestions.