Having received the nuts I ordered I was worried it would be necessary to mess about recutting the slots as on paper the nut was wider than the existing one. In practice it's pretty much a perfect fit even if it is way too tall.
I could in principle have messed about measuring the string to string spacing and distance from the edge of the fretboard but when it comes down to it, the human brain is quite good at spotting when things are evenly spaced and/or parallel. It's close enough for me.
Playing the guitar a little with it like this the nut is so high it's noticeably out of tune at the first few frets. As you fret the note you're moving the string so far it's like a small string bend. It'd probably be good for playing slide though.
So I've glued the nut and left the strings at tension while it dries to keep it in place.
Cutting the nut slots deep enough then finishing it off is going to be a bit of a chore. I've got some nut files but I'll probably have to start with a fine saw.
I did start off sanding some material from the bottom of the nut but bottled it after a while as I got worried I'd end up with the bottom being no longer flat and square. Taking material off the top of the nut afterwards means I won't need to worry about that.