What should the regulator read for a Daystate Huntsman Revere at 12ft lbs. in .177?

I assume you are not altering the hammer spring, so the better way to approach it might be, adjust the reg to the set point required to yield 12 fpe, whatever that might be. Given the Daystate hammer system, it might be tricky to attempt a balanced tune in the traditional manner. Daystate would probably recommend not tinkering with it.