I am testing the accuracy, for my location, of a Suunto magnetic
compass by sighting Polaris during its lower culmination. Upper
culmination cannot be used because of daylight.

Are these working assumptions true?

1.
An online reference gives the TUC (time of upper culmination) in
Universal Time (UT):

http://www.cadastral.com/2004mar.htm

Now, this is important: Is this the TUC at the Greenwich Meridian?
(No TLC, or time of lower culmination, is given.) Reference mentions
no actual location.

2.
TLC is approximately 12 hours before or after a TUC.

("Approximately", because between two sequential TLC's there is a 23h
56min 2.9sec interval, due to Earth procession in solar orbit, an
"offset" of about 4min.)

3.
Therefore, TUC will occur about 11 hrs and 58 minutes after a TLC.
(Dividing the above offset by 2, and subtracting that from 12hrs.)

4.
My longitude is W 89d 13m 45s. This works out to 5h56m52sec absolute
time difference from Greenwich Observatory, based on 15 degrees = 1
hr. (360degrees divided by 24hrs.)

5.
Therefore, for my longitude, a close value for the TLC after the
reference TUC is:

Greenwich TUC, plus ~11h 58m, then minus ~5h 57m.