Best neck radius for slide

[sandy]

Hi Folks, any one got any thoughts on the best neck radius for playing slide.
Is it better to be flatish like a 12 or does the camber of a 7.5 help on the first 3 strings. I am currently sitting in the middle on 9.5 but I was curious to know the thoughts and preferences of other players.
Thank you.

[grenade]

After playing a lap steel I like flatter, When my chrome reso needs fretwork it will get sanded from a 12" to a 16".