Which Xgauge code are you using for the boost? The following might help you get rid of the negative reading, it did on mine. You will notice the MAP and Boost are the same except once you enter MAP get reading first thing engine cold and see what it says. (14.? ) Then enter the same code except enter the last 4 digits of the MATH line with which ever FF?? corresponds the MAP reading.
Manifold Absolute Pressure
TXD: 07E0221440
RXF: 046285140640
RXD: 3010
MTH: 012203200000
NAM: MAP
Boost
TXD: 07E0221440
RXF: 046285140640
RXD: 3010
MTH: 01220320 (0000)<<<<these 4 would become FF?? from below.
NAM: BST
With a little tweaking you can adjust this to read boost as you said.
To do this, just plug in the MAP code - get a reading from it with the key in the on position but the engine off, then based on the reading changed the last 4 digits of the MTH to whatever corresponds to your reading on this table
14.0 -- FF74 (I USE THE "FF71" FOR MY AREA, IT WORKS GREAT)
14.1 -- FF73
14.2 -- FF72
14.3 -- FF71
14.4 -- FF70
14.5 -- FF6F
14.6 -- FF6E
14.7 -- FF6D
14.8 -- FF6C
14.9 -- FF6B
15.0 -- FF6A