I actually wanted a 48.5mm nut but I was fooled by
the specifications for the X20 Nylon Artisan on Emerald's website. It says: Nut Width 1 7/8", 48.5mm. Being a metric guy I focused on the latter but copied both in my specs to Kevin.
Unfortunately 1 7/8" is closer to 47.6mm and that's what I got. Now 0.9mm extra may not seem that much but it could probably have eased the high E-string slippage.
Apart from slippage this nut width also poses a higher risk of accidentally muting an open 1st string if one does not position the left hand in the classical way. IMO a crossover nylon guitar should allow for both classical and steel string hand positions.