20-25% is a rule of thumb. Mine actually runs around 22%. Depending upon the age of you 250/2500 will greatly effect the amount of pin weight you can carry. New trucks have a much higher rating than say 6 years ago. Don't forget to factor in the weight of the hitch (200#?). Guess I would look at the GVWR of the trailer (dry weight is useless)and take 22.5% of that. If the truck will carry that your good to go. Example: say your truck can carry 2,000# after you add the family, fuel, hitch etc.... 8,000# trailer x 22.5% = 1,800# pin.
If you don't have the truck, many will find the trailer they like then buy the truck to match (what I did). They make 5r's that a 150/1500 is suppose to be able to pull so you can probably find what you want. Many these days seem to run in the 12,000 - 16,000# GVWR range. Maybe a 350/3500 SRW would be more applicable.
Happy hunting :)