so someone do a search (I am way to lazy as I don't really care) on the actual poundage of the 75mm HE round.
I know it has been tested in game and someone had a chart of all the different rounds and what = 100lbs of explosive... IIRC that was the baseline, so and so many rounds of .50 or 20mm =100lbs, and tank rounds were brought in, rockets too giving the approximate "value" in AH.
I'd also like to see if all arenas have the same hardness settings for objects, just out of dumb waste time curiosity.
BTW the calculations if your numbers are correct, make the 75mm worth 148lbs of explosive (for an HQ that actually has a hardness of 37,000lbs)