I always cut my slots so that the bearing surface is at the fingerboard side of the nut. I angle down a little more than the angle of the headstock towards the headstock side so the slot is slightly falling away as the string leaves it on its way to the tuner. Buzzing, in my experience, has more to do with the profile of the slot than anything else. I try to finish the slot with a very fine needle file to give a round profile to the slot which is the best. The slot width should, as far as possible, match the string diameter.

A square profile, e.g. as given by a fine luthier's fret saw (I have them in several widths), can be OK, especially for unwound strings but I will still try to get the finest part of my needle file in to round the slot if possible.

A V-shaped profile, e.g. one cut with a V-profile file, tends to buzz the most as the wound strings have two very narrow bearing surfaces and strings can on occasion jump out of these depending on the geometry of the headstock and depth of the nut.

The ideal depth, if it is possible, is such that the wound strings sit so half of their diameter is in the slot and half above. Squeaks can be reduced by applying a little graphite from a sharp pencil into the slots.

Hope some of this is understandable and helpful!

Peace

moo

(trained and ex-fulltime intrument repairman)