As long as you are getting a consistent reading, it doesn't really matter how accurate the gauge is. A steady reading is the main thing. You will learn where it runs normally, and if it goes out of that range, then you will know you have a problem.
Not what you wanted to hear, but JMHO.
Happy Glamping, Norman & Elna. 2008 Winnebago Adventurer 38J, W24, dozens of small thirsty ponies. Retired after 40 years wrenching on trucks! 2010 Ford Ranger toad with bicycles or KLR 650 in the back. Easy to spot an RVer, they always walk around with a screwdriver or wrench in one hand!