The main point was to measure the bottle temp and look up on a chart what the pressure is. That will tell you which gauge is correct (or off the least).
Thereˇs a graph on lt1tech that looks good:
https://ls1tech.com/forums/nitrous-o...ure-graph.html
Virtually identical to the one I put together on Excel 25 years ago. Can't find the Excel file, but found a printed sheet in a loose leaf notebook. I can give you a copy, or send you a .pdf.
That 20+ year old bottle could have been flowing poorly. I remember one pull at 2nd St right after I ditched the Mufflex 4” catback where it lost 100 HP on a 300-shot. Lon pulled the straight pipes/turndowns off…. still down 100. Refilled the bottle in a fridge, heated the full bottle up, still down 100. Substituted one of 2nd St bottles, and the 100 HP reappeared. Appeared something went wrong in the bottle valve assembly. Might explain the 7# left in the 20+ year old bottle.