I have this gage, bought it from Keystoker for my K-6 boiler, excellent gage and easy to use.
Bacharach model# 13-19008 "Draftrite". Phone 724-334-5000
Has a 6" tube (approx) and a hole on the tube side.
To use the gage -
Put your finger over the hole and insert the tube about halfway into the draft hole (on my fire door on my K-6 unit).
Level the gage so that it reads zero, Holding the gage steady, remove your finger from the hole and note the reading. Negative pressure is to the right of zero, that's what you want.
The scale on the gage is 4, 8, 12 - these are -.04, -.08, -.12. Ther are smaller increments of .02, .06, and .10 but they're just not numbered.
My unit runs at -.02 over fire, -.04 idling (when stoker is idle and timer is on). Don't want to get too close to zero or you'll get gas coming out of the stove into the room.
IM me with any other questions.