Not sure if this helps, but here is how iTunes has it implemented (not that it's better, just a different perspective). This behaves the same for MP3 & M4A alike.
- You can only select Track # or Disc #, you cannot select the totals
- Both are displayed as # if no 'total' is present or #/## if one is
Get Info dialog
- You can see and set all 4 fields independently via 4 text boxes. Two are for Track Number & two are for Disc Number, both sets of boxes are seperated by 'of'.
COM interface (only reason I like iTunes)
- You can get or set all 4 fields independently via the TrackCount & Track Number or DiscCount & DiscNumber properties
Now, with that said, here's what I'm seeing:
iTunes: Update Track Number (two fields) to 1 of 10 & Disc Number (two fields) to 1 of 1
MP3Tag: %track% = 1/10 & %discnumber% = 1/1, both %totaltracks% & %totaldiscs% are blank
same result as MP3
MP3Tag: Update %track% = 1, %totaltracks% = 10 & %discnumber% = 1, %totaldiscs% = 10
iTunes: Track Number = 1 of & Disc Number = 1 of
MP3Tag: same as MP3
iTunes: Track Number = 1 of 10 & Disc Number = 1 of 1