The first (blue) measurement is the correct diagonal at Gauge. The next is the correct measurement if you're 10 mm low. The smaller red Error number is the Plumb Error for this course at this gauge error. If the Error measurement becomes significant, use the appropriate diagonal measurement for your gauge error.

Pull your tape from the origin point and using a fine tipped white marker, mark the correct measurement for each course. Lay the blocks with their centre top to the tape mark.

As any deviation to gauge will cause plumb errors, the sheet lists correct measurements for variance in gauge, as well as plumb errors for gauge variance.

Check the gauge occasionally and if the plumb error is significant, use the alternate measurement on the sheet. The calculator sets the circle or arc out to suit blocks, giving the inside and outside perp measurements and allowing you to open or tighten the perps (bond) in quarter block increments to a suitable size.
It calculates the 'block + perp' outside measurement, so you can check your bond as you go around. E.g. if the outside 'block + perp' is 418 mm, and you've laid 10 blocks on the way around, 10 X 418 is 4180. Pull a tape around the outside of the arc to see if you're 'back' or 'forward'.