HI
Got ya, but with the jig, it will do all the work for you

,, no need to calculate the diameter , use the jig to cut the circle out,plus put the trim lip in place all with one jig, just mark the center point, screw the jig down , to get the job done..
But here's a link that may help
http://www.routerforums.com/email-ro...ing-chart.html
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