I'd say, don't overthink this! DCC is made to be unfussy! If there is a little variation in voltage from block to block and booster to booster, decoders with back EMF will compensate and your loco speeds should be almost constant. It's unlikely you'll notice any effect.
My EasyDCC boosters, and I suspect many or most others, use low resistance MOSFET switches to alternate the rail polarities to convert the DC from their external power supplies to the track DCC. The track voltages will usually be only a tenth or so of a volt lower than the DC into the boosters.
Setting the DC power supplies to approximately 15V is close enough. If you have any current sensing diodes or resistors in the wiring between the boosters and the rails, they will reduce that voltage a bit, but your decoders and equipment should run just fine. And, the power supplies' DC voltages to the boosters are easily measured, with no need to worry about meter frequency response.
Similarly, the DC input current to the boosters should be just a little bit more than the DCC current to the tracks. A little bit is needed internally to power the boosters' circuits, but not a lot compared to boosters' track current ratings.?Again, no fancy meter is needed to measure DC Amperes into the boosters.
