Surface area is calculated by square footage of the fuel's surface (LxW=FT²).
It is rare that a surface area will have an actual square shape except for containment areas underneath storage tanks.
Square footage is an educated best guess with no measuring tape.

Simple math for water supply needs:
This is only based on the application rate needs.
Whatever foam eductor/nozzle combinations are used = water supply must be maintained for the duration
App Rate 125gpm = Water Supply 125gpm

Use anything familiar to judge area:
Attic ladder x attic ladder (1 dump tank )(10'L x 10'W=100 FT²)

10 dump tanks (100Ft2 each) = 1,000 FT²

Crosslay by a yardstick (200'L x 3'W=600 FT²)

Squad 6 appx size (36'L x 8'W=288 FT²)

1 hose length by 1 hose length (50'L x 50'W=2,500 FT².)

Crosslay by 1 hose length (200'L x 50'W=10,000 FT².)

Crosslay by Crosslay (200'L x 200'W=40,000 FT²)

Or use any combination of objects for irregular shapes

You must train yourself for estimating square footage or ......
determining foam needed is an uneducated guess with potential consequences!

Now the Shell station