I agree totally with Dan. If you want a chance in PHRF, buy a class boat that does well. A one-off or unknown will tend to get a low rating. If you want to sharpen your racing skills, go one design. Some of my best racing memories are in a Lightning - and a wooden one at that! Nothing like beating the hot fiberglass boats in an old woodie.
I ran some informal races (Power Squadron) where the RC (me) would assign PHRF nos. for boats that didn't have them. Some skippers would try to enter without PHRF numbers, when I knew damn well they had them from other racing they'd done. Others didn't seem to have a clue (or pretended not to), trying to enter a boat with a rather vague description. So I'd ask a question such as "Does your boat have a stripe on the side?" Then I'd see their reaction to a 10 sec/mi penalty for the stripe. Some didn't care and some went ballistic. I never did penalize for the stripe, but those that didn't care seemed to get higher ratings than those that went ballistic. I always figured if they were serious about racing, they would get their boat rated by the local PHRF folks. My personal ratings for an unknown boat always tended to be low and unfavorable.