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If you need to make several repetitive crosscuts, but you don't have a power miter saw, you can still get very accurate results using a circular saw and a speed square.
Use the speed square to draw your cut line and then use that same speed square as an edge guide. Most saws have a notch that shows exactly where the blade is located.
Place this notch on the cut line and the edge of the base up against the speed square and then make your cut.