Promash isn't really predicting a final gravity for you. That's based on your strike temp, yeast selection and the ingredients you choose. If I miss a mash temp I record it in the notes section of the brewing session I'm in and then adjust the mash tun thermal mass. Now that I have the mash temp pretty close, sometimes I miss by a degree, I simply record in my notes and wait to see the effect on the end product. If I see a trend over 3 or 4 brews, then I might go back and tweak the mash tun mass, just to try and nail it a little closer. 0.38 is not a bad number to start with someone once said. It worked pretty darn close for my pumped setup, but I'm now using 0.33. All depends on your setup.