So in a nutshell - Boost, normally expressed in in.Hg, is MAP minus (MAP at a nominal cruise power setting). So idle is something like -8 Boost, and combat power was around +10 boost, expressed in in.Hg.
It was used, as MAP is, for setting power in general preference to using RPM. The Hurricane 1 did not have a constant speed prop - the earliest models had a fixed pitch prop, the later models had a variable-fixed pitch prop with a choice of two or three pitch settings. So a pitch setting was made, then the powerplant was controlled on the throttle, primarily by reference to boost pressure, rather than RPM.
Have I got that right?
G