How do I cut an arc in a pine board?
Whenever I want an arc that accurate I use my 3hp router (a smaller rated router will do too, just for smaller scale work) screwed to a shop made pivot (trammel) and make repeated passes, next pass deeper than the last to complete the cut.
A circular saw may make that tight of a curve if you set the saw shallow enough to just cut through the material. It will be quite rough, not recommended for finish work
Here is the formula to figure the radius of a given arc.