OK,
Assuming it is a GN engine with 8:1cr, you are looking at 14.7psi (at lea level) x 8.0(cr) x 1.1 (for each time the engine turns over on the comp stroke which should be 4 times to get 150#) - 14.7 (correcting for the gauge reading 0 to begin with) = 150#
example (14.7x8x1.1x1.1x1.1x1.1)-14.7=150psi
now add boost of 25 psi and it would be 14.7+25=39.7
example (39.7x8x1.1x1.1x1.1x1.1)-14.7=450.298psi
now assume that you will get more blowby with higher cylinder pressures and will also bleed more off between Comp strokes.
example (39.7x8)-14.7=302.9psi
now I am not sure what the real formula is for a running engine under boost since it depends mainly on the effiency of the engine, but the above numbers should give you a ballpark figure. I know the first formula has always been pretty close when I have done compression checks turning the engine over 4 times.
HTH
BTW 302.9psi is roughly equal to 20:1cr