It seems low.
There is a formula by the U.S. Coast Guard that I've seen online.
Transom height, waterline, phase of the moon, color of the neighbors dog house, ex wife's shoe size, etc.
Google it.
Good luck.
Dan'l
Try this:
2003 BoatBuilder's Handbook | Safe Loading Section - USCG Boating Safety
PDFUSCG Boating Safety › builders-handbook
It's a PDF but the formula is there.