The miter saw is one of the first power tools I bought. It is still one of the most used tools in my workshop.
This is to make sure that the blade is exactly perpendicular to the table. It helps to get straight cleans across the thickness of the board with no bevels.
You can easily accommodate all the cuts needed in one single 2×4 sheet of plywood. Or, you might have enough scrap plywood to build this.
If you use a speed square and check the ends, you will almost always see
Once you have measured and marked the board, use a speed square to draw a line across the board using a sharp pencil.