Crank
The MP guage only indicates 30 in at sea level and on a standard day. You will see less MP (much less at high/hot airports) in other conditions, Therefore it is always a good idea to compare the field barometric indication (ie what the MP guage says when the engine is stopped) with what is showing on the MP guage when full power is achieved at the beginning of the takeoff roll.
However full power in an aircraft with a constant speed prop is only achieved at field baro MP (minus 1/2 in for induction losses) and redline RPM. If the engine has a dead mag it will not make full RPM in the takeoff roll because the prop remains on the mechanical fixed pitch stops in the full fine position untill about rotation speed.