Phono Preamp Gain

You cannot really extrapolate from one system to another to estimate the needed phono stage gain, especially if the line stage is passive. But amplifier input sensitivity, and speaker efficiency also play important roles. Anyway, the OP cannot break anything by trying different gain settings. And it seems likely that somewhere between 42 and 46d B gain is going to be ideal.

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.