Cutting string slots on nut for bass, need some pointers

[Bjorn.LaSanche]

I am replacing the nut on my bassists abomination of a stringed instrument.  the previous one I think was made out of more dried CA, than bone.

I have the nut so it sits in the slot with a nice tight fit, made the fret board radius mark in pencil. Drew a second line above the first showing me the fret height off the fret board.  Now before I screw this up, I would like to draw a line that shows how far above the fret height, I should use as a stopping point of filing the string slot BEFORE I start the fine tuning filing of the slot.  Most places I have looked at give advice to measure the height of the fret wire and roughly .015"-.020" above the fret board radius line as a good starting point.  Meaning ok you can hog out nut material to here then stop, change mental focus to detail work and start finalizing shape, slot angle, etc..  These have been all for guitar and I have used them on several guitars I have replaced nuts on and works quite well for me.  This is the first bass I have done.  because this bass had the shitty nut from hell I really could not get a good reference of the distance between the top of the fret wire and the bottom of the strings to judge where to stop the main material removal and change to detailed working metal state.  Is it the same as on a guitar?  Seems that this is the elusive bit of information that no one discusses when trying to research this for bass guitars.  I would use a Tusq pre slotted, but this guitar's neck is wider than any pre slotted bass nuts available.  Thank you guys for any pointers.  I am just trying to get the nut into a rough finish from straight blank, but well before nearing finish