I guess Kirk's chair
is on grid 3C42 but the other parts of the bridge is on 3F100 ?
Now that I'm almost finished with the sketches for Decks 5 (revised), 6 and 7 and finally arrived at alignment proposals for the main corridors that work out fine (IMHO) I'm able to propose an alternate suggestion.
Starting at the starboard stern and moving counter-clockwise I have Transporter Room # 1 and # 2 on Deck 6, # 3 on Deck 5, # 4 on Deck 6 and # 5 on Deck 5.
I noticed that the shortest evacuation route from Kirk's cabin 3
F 121 in "Journey to Babel" would lead to transporter room # 3 while the shortest route from the isolation bedroom 5
R 671 would lead to transporter room # 5.
Therefore the first number might merely indicate which transporter room is the assigned one in case of evacuating the ship by the means of transporters (like they did in "This Side of Paradise").
The alphabetic letter might still indicate a power grid or else.