Pretty sure each dungeon is weighted by the dungeon itself. Not a percentage of your level but a static amount based on your level and the dungeon in question.

I did tam-Tara at level 16 but only about 2k from 17. I hit 17 after the first pull. I hit 18 after the second demon. And I hit 19 upon completion. I don't remember the exact numbers in question, but I clearly got more than 45% throughout.

As for arkista's reasoning about abandoning higher level dungeons. Your logic is flawed. Because there are more resources to consider than just the lump sum at the end. The number of mobs in a given dungeon, the time spent in the queue, the amount of time you intend to spend in dungeons in a given day all factor in.

One thing I would particularly point out is cutters cry has a good deal more mobs than vigil does. You get a fair amount in the dungeon itself.