Hi Alan, what I'm talking about is the string break angle over the zero fret, by making it lower it reduces the friction and gives a more stable tuning.

this shows my string break angle.
I know the plans state 17 degs for the headstock angle as you can see I didn't do that mine was about 5 deg just be wary of the zero fret height and the point where the string goes through the tuners as this is relevant to the actual string angle.
I was aware that somewhere in Ken Parker's notes that he uses a 4.5 deg string break angle.
As to the sideways slip

When trying to bend a note in the first fret area the string deflection sideways between the zero and the nut will cause the string to slide along that zero fret a little and you can feel it under you finger, it doesn't feel natural to me like it would on a standard type nut, it doesn't sound any different to normal it's just the feel.
Hope that explains it, cheers Mark