Old tube mics were not intended to distort. They were designed as linear and clean as possible, but engineers found that a rock singer screaming into a mic at 3 inches sounded much louder than the actual level on disk, as the tube was pushed slightly into clipping.

400mV is a reasonable level for 0.5% 2nd harmonic with a LDC. It should sound like a good tube mic. If it is too clean, then you can experiment. Large capsules I have measured are more than 56pF, around 80pF out of circuit, and just over 100pF in circuit with 60V bias. As for sensitivity, feeding a transformer-less charge amp with a 100pF dummy capsule:
Gain vs feedback cap, no cap = 0dB;?
10pF = -3.4dB; 25pF = -5.9dB; 48pF = -8.7dB; 68pF = -10.7dB; 115pF = -13.4dB.
@100mV RMS (280mV p/p) out w/10pF, 2nd harmonic = 0.5%; @ 10mV out, 2nd = 0.05%

It will be interesting to see if P@t’s mic measures similarly to mine, which is like a KM-84 with a Schoeps PNP output stage instead of a transformer. I have no idea how much of the distortion in my mic is due to the charge amp and how much is FET since the FET is also the charge amp. Your mic has the two circuits separate, so you can experiment if you want. Bypass the FET and see how different ratios of feedback work. Try different bias points and load resistors on the FET while keeping the input stage fixed. Try different FETs.?

Or just find something which sounds good and go with it. I’d build one if I didn’t have 3 or 4 circuits ahead of it to try out.

Regards,

Henry