Many thanks for the replies. So, if my understanding is correct (and the screws are normal R/H thread), the adjustment procedure would be…
1. Loosen both outer screws a small amount.
2. Perform the required adjustment using the inner screws (unscrew to raise vial holder that side).
3. Tighten the outer screws and re-check the calibration.
4. Repeat 1 – 3 as necessary.
Gary