If you take one foot (1.00 feet) and divide by 12 inches, you find each inch is 0.083 feet. (Figure .08 get for simplicity).
How far is the lake down at 503.78? Subtract 500.00 from 500.32 = .32 . It's another .22 to get to 503.78. .added together .54 ÷ .08 = 6 inches plus, or almost 7 inches.
I use this site
https://www4.des.state.nh.us/rti_data/WEIN3_TABLE.HTML . Updates regularly during the day.
Dave