I've always heard that 7-10% and have yet to ever own a trailer that puts that much load on the hitch or have a trailer that acts squirrely because it only has 5%
Agree, but I've seen some trailers with little to no tongue weight, even negative tongue weight when loaded improperly. Both caused issues.
I have side tracked this thread as tongue weight isn't vitally important. The OP needs to find out where the vehicle sits when the trailer is connected and select the proper tow bar.
EDIT: Which looks like it has been done... Happy boating!