Direct comparison to strain gage

I'm modeling a simple fixed-free beam to compare with a piezo strain gage with a foil strain gage that I have on an actual beam (aluminum, 20cm x 2cm x 2mm). I'm trying to decide which strain I should have Comsol give me that would directly compare with the physical strain gages. I have the foil strain gage mounted on top of the beam near the fixed end and the piezo gage directly underneath it as well as a force sensor on the free end where I have a shaker.

I apply a force similar to what my force sensor gives me (~41N) to the free end in the Comsol model and want to see the theoretical strain on top and bottom where the physical sensors are.

I was using the "First principal strain" for my results which looks alright on the top of the beam, but I don't see the negative of it on the bottom as I was expecting. Is this the wrong strain to look at?

I'm using solid mechanics in 4.3a

Thanks for any help
Jon