You'll just have to calculate it yourself. Thermal mass equations are hard to translate from one system to another unless they are exactly the same. Your mash tun likely has reflectix on it which changes the equation (I remember their systems had that too).
Better off just figuring out thermal mass with water on your own and then plug it into the software.
My tuns are all the same size too and I wish my HLT was another 1/3 bigger. So, after you strike, fill up and heat the HLT again right away for sparging and subsequent cleaning.