I tried out a different version of the charge measurement. I'll detail it more later but the gist of it is that I drive the pitch of the TM using the built-in frontend lockin and feed back the lockin output to an offset on the ESD quadrant drive. This is done without any DC bias on the full face bias electrode.

In any case the effective offset voltage I measure for the four ESD quadrants are:

LR: 184V

LL: 159V

UR: 1V

UL: 12V