After some testing I am pretty sure that these values are indices for a lookup table.
The difference between the values for the same quality seems to have something to do with crafted and looted items. Not sure what the other 2 or 3 values are used for though. The charges shouldn't have anything to do with this field. They are controlled by the 20th field (
see wiki).
e.g.
364 - gold vr16 looted
370 - gold vr16 crafted
375 - gold vr16, damage is 0 when level field is 0 - no idea what this is for
Dagger of Agility
|H1:item:69287:364:50:0:0:0:0:0:0:0:0:0:0:0:0:25:0:0:0:498:0|h|h
Magnus' Inferno Staff
|H1:item:56321:370:50:0:0:0:0:0:0:0:0:0:0:0:0:29:1:0:0:0:0|h|h
From what I have seen so far I think your staff would become 361 when you improve it.