82.83 gallons in 39" of height equates to .47" per gallon

.47" times 58.98g = 27.7"

82.23 gallon capacity / 39" height = 2.1 gallon per inch. That's at 0% expansion.

At 4% expansion, the volume drops to 78.9 gallons total capacity and BeerSmith accounts for that. This is useful when actually brewing, but when designing equipment a difference of even a few gallons can be problematic.

The issue wasn't what the mashtun would hold, it was what depth a heat coil should be installed for optimal use. In that case, converting volume to weight takes the heat and displacement out of the equation.

I'm all for making it simpler to calculate, but I'm not at his brewery, I can't see the equipment and I don't know his methods. Having worked with American, German and Chinese brewery manufacturers, I've found that knowing the root math helps avoid design mistakes from assumptions. It really sucks to find out after 4 months of waiting and hundreds of thousands of dollars that capacity is even 2% less because one piece was approved with the wrong dimensions. An 80 bbl brewhouse with a 2% error in capacity can add up to 4500 bbl/year of loss or $1.7 million in lost potential sales per year at capacity. Basically, the cost of the whole brewhouse, every year. On the scale of OP, it could mean $15K per year, which aint nuthin.