As far as zeroing the gauge, I can't see any need for getting too carried away with this. The greatest 'error' or change from zero that I've seen is about.01". That requires me to squint to see on the gauge. It's about 1/16" of the red fluid.
If you have a manometer on a stove in a tight house, watch it while you have someone turn on the exhaust fans in the bathroom [s] or the exhaust vent fans over the stove. I bet you'll see a change in the reading. Or when the gas or oil hot water heater kicks on and pulls a draft on the house. Or The clothes dryer. I think a person could 'turn grey' trying to chase the minor fluctuations.
OK, I admit, I'm not very 'anal' about these things, maybe this is from spending 35 years looking at about 30-50 gauges on an airplane panel. Most of which never change, and if they do you can't do much about it.
Unless the manometer is showing some really way-off reading, it is just a general 'health-check' on the chimney or powerventing device.
The difference between .03" and .05" I just can't see as significant day to day, unless a continuing downward trend is noticed.
When I walk into my boiler room, I take a look at the manometer, if it is reading .03", I can be assured the boiler hasn't fed fresh fuel into the stoker recently, the chimney is cool and not drafting as strong. If it is reading .07", there is a lot of heat in the boiler and chimney, creating a lot of draft.
I have a temp probe in the chimney too, and have confirmed these observations. My temp probe is a dual reading wireless sender. The reciever is in the house, where I can look out the kitchen window, see if the light that comes on with the stoker motor is on, then watch the flue temps and water temps.
The manometer is probably a good tool for monitoring the cleanliness of a motorized draft inducer, power vent, or other device. Allowing you to monitor it's performance with out having to disassemble it to inspect for ash. Once the performance drops, it's time to clean it. But this should happen over several weeks or months, ot days
Just my take on the manometer. Greg L