ShowSMPTE() drop-frame algorithms

[fvisagie]

I'm looking for the algorithms used to implement ShowSMPTE's drop-frame counting. It's help file covers NTSC TV, but none of the others:

Something similar holds also for the following framerates:
framerate range base-rate
[47.951, 47.953] 48
[59.939, 59.941] 60
[119.879, 119.881] 120

I've found some discussion of NTSC film rate and double NTSC film rate, but nothing that indicates what was eventually decided upon.

Any ideas how those were implemented?