Without knowing exactly how you missed it - too high, too low, by how much - I'm assuming you were low. If that's the case, here's one solution:
I'm not sure how Beersmith does it, but if it has a thermal mass input, you can "determine" the thermal mass using an empty mash tun and 170 deg water plus some other steps.
However, using a constant (when other variables are not constant) is not always going to produce the most consistent results. Try this instead - calculate your strike water temp as usual and then shoot for a few degrees higher. Dump it in the mash tun and close the lid. Check the temp over the next few minutes. In those few miinutes, the tun will absorb some of the heat, heating the tun and dropping the temp to your desired strike temp. At that point, dump in the grains and it shouldn't drop anymore.
The best way to determine what works is to always take accurate readings of all your steps (the grain temp, the tun temp, ambient room temp, etc) and adjust as you need to. It should only take a few brews to dial in your system.
