I'm not 100% sure how skimmer ratings are calculated to be honest
The only method I know of is to calculate the skimmers GPH rating:
4 turnovers per hour x tanks capacity in gallons = GPH rating. How they go from this rating to actual gallons in the tank, I'm not too sure. I guess if you knew the GPH rating of your pump you could solve for the tank capacity, but I'm not sure if it is as simple as that (but that's what I do).