Phono Preamp Gain

You don't really need to worry about any math equations, because you have a combination that cannot fail to provide enough gain up the chain, and because you cannot damage anything by experimentation. But if you insist, there are excellent calculators for converting signal voltage to db and vice-versa, on the internet.  There may  be one on the KAB website, for example.  For what it's worth, I am running an MI cartridge with output similar to that of your cartridge into a Manley Steelhead, with no added gain downstream from any linestage.  I find the 55 db setting on the Steelhead works fine.  My amplifiers seem to have a very high input sensitivity (meaning, full output is achieved with much less than 1V input), which helps. In your case, because you do have an active linestage section that adds 12db, according to others who posted here, probably 48 or 50db is a good choice.

I tried all the recommended MM levels on my Schiit Mani and Freya with a Grado MM. Two levels sounded good. One was a better match for being similar to my digital rigs channel, so I'm using that. (I should note there was a distortion before the unit and cables burned in, so if yours is a new set up, do a burn in on the Mani).

"I’m not sure if the JBLs are anywhat of a good match for the Primaluna but I got them for dirt cheap at an estate sale so they are staying for a while."

nmo25- I bet the JBL's will sound good, if you like the "vintage sound"

Vintage sound to me is not overly detailed, overall presentation leaning on the warm side. There shouldn't be any lack of bass with the JBL's.

You might be able to fine tune the JBL's tone with the 2 gain tubes on the PL.

@tablejockey re: "There shouldn't be any lack of bass with the JBL's." You might want to edit that to say 'mid-bass' as the L100 starts rolling off steeply around 55Hz. It's also a high-Q design with a peak of 6dB or more so an octave above the port tuning frequency. Low bass was never a capability. 

I am using a SS Carmen II, which is the same output voltage as your cart. (2.12 mV) and running it into an SPL Phonos using the 46 db gain setting. Works great, plenty of headroom without overloading. I'm using a passive (Hattor Audio) preamp so no extra gain there.