A couple of racers have asked me recently about the formula that is used to calculate the time correction using PHRFs.   The formula is Elapsed Time x Time Correction Factor (TCF=A/B+PHRF) equals Corrected Time.  B is 550 and A is 700.  Both numbers can be adjusted up or down according to wind strength, but I usually leave them alone.
More detailed information can be found at   http://offshore.ussailing.org/PHRF/Time-On-Time_Scoring.htm

Mike Carron, Vice Commodore