Unless you cut a lot of non-standard sizes, it's often easier to buy mat board pre-cut. You can get many standard sizes and they'll have perfect straight cuts and square corners. I use 14x18, 16x20, 20x24, 22x28 and 30x40. I get them from Redimat. They have a large selection of rag boards (I use Alphrag) and colors.
Save your rotary trimmer for photo paper.
Best,
Doremus
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