Lookup mmdvmcal in RSSI mode (S/s).
It's included in Pi-Star.
It will give you the RSSI number for a given RF input.
Just hook your Sig Gen to the RX and adjust output in 10db steps from no signal (< -130dbm)
until it saturates maybe around -50dbm.
Enter those numbers in RSSI.dat.
If someone has already done this for your model or radio it's probably close enough but easy to check.
Look at the mmdvmcal number with no signal, then TX with an HT nearby and compare that to the upper end.
MMDVM_HS Hat hardware