How are fuel cells officially measured, inside or outside dimensions?

Assuming a tank is not obviously possibly oversized and an actual capacity test would be done, is capacity calculated from the outside dimensions or inside dimensions?
I have a person wanting to buy my cell to put on a GL1800 with native 6.6 gallon capacity.

My cell is made from .125 aluminum with a center baffle. Outside dimensions put it at around 4.89 gallons but subtracting the thickness of the material puts it at about 4.53 gallons. They are wanting to buy it for Butt Lite but I don't want to sell something to them that might cause issues. The 4.89 gallon measurement would be right against the 11.5 total and that's not including fuel line and fuel pump capacity. Thanks!