I don't have an answer for you, but my tank is about the same. I've gotten 9 gallons in it, and it was dangerously high...like don't jump near it or it will spill over. When they calculate the gallons they're going from the seam and edge at the very top to the very base...and no they don't hold quite as much. I think the best way is to measure the band at the top and find out how many cubic inches that takes away from the volume of the tank (there are conversion charts to find the specific volume to cubic inch mathematics). I've found it to be at least 8% water loss for both my friend's tank and mine. So I would think that's standard loss when accounting for the water. Also if you're using sand and a substrate, you have to account for the volume of water displaced by that also.
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