is there any way to calibrate the center "phantom" channel level comparing to the mains left and right?
Use JRSS and check "move center to front L/R".
Click Load/Save at the bottom right of DSP Studio and save the DSP as JRSS Phantom Center.
Uncheck "move center to front L/R". (I still recommend leaving JRSS on to upmix 5.1 content to 7.1, but you can also turn it off if you want. If you don't uncheck the phantom center first, I think there is a bug that it will still be phantom even when turning off JRSS.)
Create the phantom center using the instructions provided by mwillems. I would add the Room Correction DSP before the PEQ to make it easier to adjust the center volume. Then just mix at 0 dB when moving the center to L/R.
Click Load/Save at the bottom right and save the DSP as Custom Phantom Center.
Go to Tools > Advanced Tools > Audio Calibration > Volume Calibration and set it to 7.1 and 48 kHz. Press play (don't press create files).
Advance to the center channel test.
Open DSP Studio and load the JRSS Phantom Center and listen to the volume level.
Now load the Custom Phantom Center. Use the Room Correction DSP and adjust center speaker level until until it sounds just like the JRSS mixing. You may have to switch back and forth between your saved DSPs until you get it close. Now you have a baseline for what is correct. You can bump up the center volume in Room Correction easily to suit your taste.