But coming back to your case: if you have no 40mm, resistance is futile. Surrender while stocks last!
I concur with Airy that 40mm feels natural. And this particular 40mm just feels right on the Zf with its aperture ring and it produces vintage like photos wide open but can be very sharp closed down.
Thank you, gentlemen, for your thoughts and encouragement. It is good to know that it complements the 35mm f/1.8S rather than competes with it. 35mm feels natural to me and so 40mm is bound to please too. As a bonus it would be, effectively, a spare lens at my favourite angle of view.
(Oh dear. I'm at the rationalizing phase
)