There are a few ways to skin that cat, first one I thought of....

Arrange it so your sine so it 'sits' on exactly half your PIC's VCC.

You can now detect zero (either in software or with a PIC comparator)......this will yield you the frequency (ie start/stop a timer when the sine crosses zero).

Now all you need to do is tuck away the same timer value when the pulse arrives ..and apply your formula.