Hi,
This is how I used to do it on the 651 (from memory):
Measure on the edge connectors of the '33 and '34 cards on the 651 backplane.
Starting with the '34 card (nearest the meter bridge). The last pins 53, 54, and 55 are 0V and are very obviously joined together - this gives you a reference point.
From pin 52 to 55 should be between -18 and -18.5V
From pin 51 to 55 should be between +18 and +18.5V
From pin 51 to 49 should be 7V - as near as you can get it
Now moving to the next card, the '33 card
From pin 47 to 55 should be +48V - somewhere between 47 and 49V.
Be careful - it is easy to get your meter probe to short circuit pins 52 and 53, or 51 and 52.
Wherever you measure it, do NOT measure and adjust between the 11V and 0V connections, ALWAYS measure and adjust between +18V and +11V to be 7V.
Ignore the fact that the service manual mentions "20V" and "13V" on the backplane connectors, that was only true for the very early (about pre 1981?) desks. It was quickly realised that 5534s didn't like being run at +-20V for very long and were dying too quickly. (Actually it might have been the TL074s on the 10 card that died...anyway still not a good thing.)
Hope that helps,
Cheers,
Andy