When looking at the 'Select Subcategory' dropdown, ZF shows as between 42-46, only a span of 4 levels; the end boss is a 48 or a 49, this should likely be 42-56 instead.

I don't know if it's a reference, but the database site I know, it says 44-54 levels at 'wowhead.com', 44-49 levels at 'icy-veins.com' and 43-50 levels at 'classicdb.ch'.
Monsters are recorded at a low of 43 to a high of 48 levels. If you average the three sites, it's about 43-51 levels.
The level of players who can enter the dungeon is 35 levels, and it is recorded in Korea as a dungeon of 40 to 50 levels.
Since many famous sites are different, it will be difficult to specify the level of dungeon based on what criteria.
If I make one criterion, I think it's going to be the lowest and the highest level of the monster we talked about earlier.
Personally, I think it would be better to let you know that there is a monster at level 43-48. Of course other dungeons should have the same standards. ;-)
Or is it the level at which you can accept the quests you perform in Dungeon? What do you think?

I personally would rather see the low/high level of the bosses, but consistency would be best. But that's a bigger question; this is more for the typo of '46' instead of likely '56' located at
https://github.com/Hoizame/AtlasLootClassic/blob/master/AtlasLootClassic_DungeonsAndRaids/data.lua#L1967

Hey maybe the lowestLvl-highestLvl is a good way.
Right now I cant remember how I set the dungeon levels :D

Just use the meeting stone, innkeeper level data that is consistent with the game data.

Instances by level
Edit to add
Dire Maul level range counts East for lower end, if showing them separately West and North should be put at 58 min level same as Scholo and Strat.

If you want to provide alternate (as an option) this might also be of use (tailored more towards dungeon exp)
Dungeons by min/max npc levels

Finally a useful metric (that usually tracks with the meeting-stone and innkeeper dialog) is level at which the lowest dungeon quest becomes available and level at which the highest dungeon quest becomes available.

That said I would simply go with the game data as baseline (meeting stone / innkeeper dialog)