Here is more than you need to know.  

https://www.ussailing.org/wp-content/uploads/2018/01/2017-North-American-Portsmouth-Yardstick-Handbook.pdf 

However, pay particular attention to this first sentence:

"The North American Portsmouth Yardstick is an empirical handicapping system meant to provide equitable scoring of race results for different boats sailing the same course." 

There are ways to come up with estimates prior to having the "empirical data" - but the bottom line is the Portsmouth ratings are really developed by boats that sail in mixed fleets at the same time on the same course over a number of years.  And of course few if any Skerry's have been sailing in few if any mixed class racing fleets over the course of years.

So, go to page 7:

"B. Handicapping Unlisted Classes Ideally, all handicaps should be verified by race data and listed in the Table of Precalculated Classes. Where unlisted classes are introduced, trial races with one or more of the Yardsticks should be conducted to determine a suitable rating in conjunction with the Portsmouth Numbers Committee. In the absence of verified race or trial data, a given boat may not have a verified D-PN handicap available. The following subparts outline two techniques to determine provisional ratings in the absence of a published handicap."

All things considered, do the calculations to the best of your (or someone else's) ability and see what you come upwith.  Having sailed many different small boats for may years, you'll find the Skerry is no speed demon.  If your fleet is informal, I'd ask the PRO (Principal Race Officer) to look at your calculations and give you a provisional rating guess and let you see how you do.  And I'd ask for a number in the range of 105.

I'd be interested in seeing what numbers you or the PRO come up with by actual calculation, and how the racing goes.  Good luck and go fast!

   Thanks for the help, but as a follow up on the initial comment, at the end of the regatta I was tired and didn’t bother to ask about the rating. Winds at 0-2 made for tough going against the current. I would have been more interested if I we did a little better (suffice to say I knew we wouldn't place). They did let us take the opti route due to our smallish sail area, so that helped. 

   I hope saying this doesn't lead to any embarrassment, and in winds that light racing is often a crap-shoot anyway - who gets the right puff at the right time (so frustrating) often wins (although there is some art to finding those puffs, but... Anyway, going up against an Opti, that is a least one instance in which I'd place good money on the Skerry.  Here's to better winds for any repeat races.

I'm 6 foot and 230 lbs..  Tried to sail an Opti once just because I never had before.  Even in the "I'm home in the tub taking a bath" sailing position I could never get comfortable.  Twenty minutes later I gave the boat back to the kid that let me borrow it, and I've never been back in one since.  But this is definetly NOT to say that youth sailing doesn't benefit GREATLY from all the highly organized and competive OPTI fleets.  Just put them in a Laser 4.7 or Radial as soon as they are ready. :)