Liquid filled gages have a little more error than one that is not. The liquid inside can heat or cool down changing the reading slightly. Each gage will read a little different. What you want is a basic target and adjust from there. Meaning if it shows 6.0 PSI on that gage and you need to remove .25 PSI you should use that same exact gage to get your 5.75 PSI reading. Dont get to carried away with if its exact or not. You are looking for repeatability of the gage when increasing or decreasing pressure from run to run. Make sure you take good care of this gage by putting it in a nice case keeping it from banging around. Gages are really touchy when your looking for .1PSI changes..
I've had nothing but troubles from liquid filled gauges. I've been flowing and with no changes made, the pressure changed. Drove me nuts until I dropped the gauge and all the liquid came out. Now it is very repeatable and works great (except for the cracks in the face).