Dimensionally accurate cut

I need to make a dimensionally accurate cut to make a spacer for my DeWalt Plunge Router that I use for rounding over projects.
Shouls cut be set for inside - outside or on path?

If it’s an insert its typically outside. But also if you are looking for the most accurate cut, be sure to measure the diameter of your bit and use that instead of the pre-defined bits. It’s rarely exactly the manufacturers nominal size.