Even though you are measurng the draft at the breach you could have insufficient over fire draft when using a MPD. Therefore we reiterate "DO NOT USE A MANUAL DAMPER ON ANY LEISURE LINE STOVE, BOILER, OR FURNACE". It is admirable to want to extract every last drop of heat but this method restricts the flow and that will cause volumetric problems with the stove that are dangerous (CO can be release in the home). Best bet is turn down the power vent so you have sufficient draft without the barometric damper flap being open excessively. Also remember to check it at idle as well as at full burn.
Please, please do not attempt the use of a manual damper in any stoker, as has already been stated not a good idea.