A guess, but....
250 ish is my guess-the newer 26's are in the 230 area, and you will have to remind them how different your boat is from the 80's vintage 25+/26's. The IB 25+ is 219, the old style 25 is 234, and your boat is slower than those by a good bit. Compare WL length, sail area and displacements and you will see.
The fastest I could imagine is 244, but I would give someone new to racing and the boat a 250 without hesitation. The Pearson 26 is also about 219, but it is similar in vintage to the 1970's 25's. The old Pearsson Commander is 249, which I think is a pretty good point of reference..
Having said that, on a 5 mile race, 12 seconds a mile is only 1 minute-which you can lose in a bad tack or a single missed shift. Considering you will act on 3-4 shifts on a given beat, I don't worry too much if my rating is 12 seconds worse than I would hope for. It doesn't take much to pick up a minute on the average PHRF fleet..
Good luck!