Increasing altitude will cause both instruments to reduce
Not so.
A supercharger will hold a set boost up to
full throttle altitude, after which it will start to reduce.
It is possible to hold MAP in a climb, but impractible. With a constant speed propeller without a turbocharger it can be done by using reduced power (rpm & MAP) in the initial climb. As altitude increases the throttle may be progressively opened, which will hold the MAP until the throttle is fully open.
Perhaps this discussion would be better by PM.
