Here's how I work with Cab-Lab 3
With the browse feature, I search for all the R121 within a library. I check them all with the "auto load" mode or whatever it is called. I usually selects two of them, one fat and dark, the other fat and neutral. That would be the "meat" of my mix.
Then, I search for all the SM57 available. Again I'll select a couple, a very bright one but not too thin, then another one, less bright. This will the "cut" part of my mix.
When it's done, I'll try different combinations of R121/SM57. I check the EQ curve in Cab-Lab, specially when I fade one IR in or out with the sliders. I always compare them to existing go-to mixes in a preset tweaked for those go-to mixes (for high gain, usually my MKIIC+ lead), not with a amp with default settings (because I won't use the amp with default settings anyway).
After that I may try to add other IRs : MD421 for a different treble content than the SM57, E906 or M160 to add some low mids when needed (I love fat low mids). I save a few different mixes, some with R121/SM57 only, others with MD412 and/or E906 and/orM160.
After that I try all those mixes again. With fresh ears, on another day, it's better. I have hundreds and hundreds of mixes in my IR test folder
Hope that helps !