Double is the standard, but I prefer three x width. Of course the type of net curtain can make a difference. I have a fine voile but if I were to buy a thick heavily laced pattern I'd buy double.
Poor porkchop is none the wiser .. no answer is right and no answer is wrong and everyone has their own idea.
We have seamstress in the house and she says ...
" As a rule .. at least one and a half times the width, dependent upon the density of the net or the voile. Not forgetting the amount of privacy required"..
More material equals more privacy .. but less daylight !
I've stopped growing them, alav. My dodgy hip means I haven't got the time or inclination to spend hours hand weeding them. I used to plant about 400 of them as I've got loads of space int allotment. I'm using the space for sowing bee friendly flowers.