Calibrating an auto level?
 

This is purely mechanical surveyor's level, for levelling only,
absolutely no electronics.
A tiny mirror or prism , on a very constrained and damped pendulum, set
in the optical path of a x28 telescope set on a spherical mount to a
tripod, to get initial rough levelling by x-y spirit bubble, then the
pendulum system corrects within the last +/-10 angle minutes of level.
It is a Sokisha B2 probably decades old, working with good repeatability
but poor direct levelling. Checking against levels off a lake it is
about -27.4mm over 100m, but consistent on return legs and repeating.
Requiring offsetting , for any non-balanced levelling, ie non-equal legs
either side of the tripod.
I've the user manual and the carry case contains the tools for adjusting
the calibration but no blurb. Cannot find a technical manual for this or
any other similar, different makes, of auto level. But I found where it
looks like you adjust , by unscrewing (not obvious it unscrews) cover
around the eyepiece.
Anyone any ideas on how to do the adjustment, assuming I have a good
long-leg sighting run to a staff. There are 4 brass set screws, holed to
take the adjustment tools , arrayed around the eypiece.
So far I've taken and recorded vernier calliper reading of each of the 4
set screw heads, across the diameter to the threaded boss diametrically
opposite, plus 4 dots of paint.
Would top and bottom ones adjust the vertical displacement?
and would the process be -1/8 turn on one and +1/8 turn on the opposite
other, measure the displacement at distance and reverse if going the
wrong way and repeat until nearer 0 displacement on the staff at the
relevent distance.
Would the other pair of adjusters have to be slackened off ,1/8 turn
say, while doing this , then finger-tip torque tightened back 1/8 turn ,
or at least balanced if not 1/8 turn available then. Its not possible to
see how this adjusment mount is constrained, or not. Hopefully no
threadlock on the set-screws or if there is, then overcome wiht a few
seconds of soldering iron.