Consensus on Water Level v. Tank Top?

While doing calculations as to how many gallons of water is actually *in* a typical tank, I realized I've never given any real thought to the best water level (distance below the rim) to use. IIRC correctly, the outside dimensions of the tank are used to make the number of gallons determination. This would yield immersed tank displacement rather than tank capacity. Subtracting an inch from width and depth and 1/2 inch from the height (allowing for glass thickness) would give capacity if filled to the rim. Of course the volume of "stuff" inside the tank must also be considered...
Any thoughts regarding optimum water level are appreciated.

90 gallon tank:
exterior dims: 48"W x 18"D x 24"H
interior dims: 47 x 17 x 23.5
Water Level 3" from top: 47 x 17 x 20.5 = 16,379.5 cu in (allow for drilled overflow plus 1" above hole and 1" below)
divided by 231 cu in = 70.9 gallons capacity

And of course, this does not account for any "stuff" inside the tank...

If you already have an overflow in the tank that will determine the water level in the tank. If you are considering adding a overflow to deliver water to a sump then the level of the box (which is adjustable) inside the tank will determine the water level in the tank. Generally I prefer the water level to be at the bottom on the brace (plastic piece added to the top rim of the tank) for a display tank. For a growout or QT water level is not as critical for me.
hths,
Pat

I have done water measurements and what I basically think is that the tank manufactures are
false advertising. JMO.

They are.