Hi Leoo,
For a basic mat, with all sided being the same thickness, the math/measurement is pretty easy.
1) Cut the outside of the mat to the final size needed. Make sure the mat has straight sides, and is a rectangle.
2) Set the the stops to the width of the mat wanted.
3) Insert a slip sheet (sacrificial board)
4) Insert the mat face down, snug against the stops.
5) Draw a starter line on one side along the track that the cutter slides on. You can also draw that line on all 4 sides if you want, but with experience they won't be necessary.
6) Rotate the mat 90 degrees clockwise.
7) Cut the mat, starting about an 1/8" past the drawn line, and ending at the stop.
8) Rotate the mat 90 degrees clockwise.
9) Cut the mat, starting about 1/8" past the previous cut, and ending at the stop.
10) Repeat steps 8 and 9 until complete.
The math/measurement is easy.
The cutting will take some practice.
I have made perfect mats with a C&H Advantage (similar to the Fletcher 2200), a straight edge, Utility knife, tape measure, and some scrap mat for a slip sheet. A Square is also useful to confirm your angles.
Mats with a weighted bottom take a bit more effort, but can be done the same way with the stops.
It's best on weighted mats to draw all sides to be cut.
