There's at least two scenarios. The first is "Getting assigned a rating" or obtaining a PHRF Certificate This requires the boat be officially measured and certified. The other is to simply research what other regions have assigned to a boat and use that as a starting guide. I believe your boat is considered a Hunter 27-2 with a Lo/Hi/Avg of 210/219/213 as spec'd on the ussailing.org but the problem is figuring out just which one they are referring to. The Portsmouth is:
Hunter 27 26'7" 7/8 (WK) HTR27N 91.2
Hunter 27 27'2" (DK) HTR27 93.6
Hunter 27 27'2" (SD) HTR27L 94.1
Hunter 27-2 HTR272 95.9
The last numbers are the base handicap ratings. (Sorry for the formatting)
Good Luck