He went with the assumption your tank was 20" wide and 13" tall like I did
120x20 = 2400sq sq inches
He assumed 2000lbs of weight
psi is pounds per sq inch so 2000/2400 = 0.83psi
With 120x13 and a more realistic weight of 1500lbs you're at 0.96psi
Happy wife = happy life
