So I've got a bone bridge that I swapped onto my tele, and it needs the slots to come down by a good amount, maybe 0.5 to 0.8 of a mm.

I bought one of these nut file sets.
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Working on the low E for some time, and while I can see some dust after a minute or two of filing there has been almost zero change to the depth.

Do I just need to spend 15-20 minutes per slot, or do I have a dud set of files? Or do they just not work well for bone?

I'd prefer a flat file set like these at stewmac, but I can't really justify the expense..