I don't think there are any rules(although I'm sure some will argue!). You have to find a compromise between aesthetics and the strength you are adding to that area. So make it thick enough to add the strength, and thin enough so it is pleasing to YOUR eyes. Good luck.
Volute's I think are a guitar/mandolin steel string builders thing for strength...most uke builders dont have them eg: Martin, Kamaka, Koaloa the only time something like a volute is done is on slotted headstocks where there is a transision to a thick headstock...The Ukes Ive played that do have these i find akward to play some chord shapes at the top end with the volute getting in the way mostly on the E7 chord...I even move the tuning pegs a little bit higher up the headstock to give a bit more hand clearance,,,, But thats just me ..but I admit the volutes do look pretty
I don't see them being needed on a uke, not like a lot of force from the headstock if the instrument gets a jolt. I see them as more being useful if an adjustable truss rod access is in the neck. They are more needed with short grain and minimal volume (thickness) in the nut area. But even then it is a tradeoff how big to make one and where it goes to. At most 25-30% of the first fret distance (just looked over to my Stratocaster, about 25%) seems a reasonable compromise.
I don’t find the volute to get in the way of playing. But then again depends on shape and how far it goes. I do like how it looks. May not matter much as far as strength as I use carbon fiber rods anyway but as with much of what I do, it is pleasing on the eye…