Tinstaafl,
Not quite correct in one aspect. As you say, a boost gauge is just a MAP gauge using a different reference. 0 psi boost equates to 30" Hg (1 atmosphere). Therefore, if you have a low pressure day, "static" boost (i.e. with the engine not running) will show slightly less than 0 psi in the same way as a MAP gauge will show less than 30", and vice versa.
IHL,
It is worth remembering that 1 atmosphere is 30" Hg and 14.7 psi. Therefore, as a rough rule of thumb 1 psi boost equates to 2" Hg. Therefore, +14 psi boost is approximately 59" MAP.
Rgds
L