I have always sprayed twice for my own peace of mind, I use the same amount for everyone, regardless what size they are. I don't waste any product and charge the same.
I may have mentioned this before but the
1st coat; covers it
2nd coat; if mainly for your own peace of mind with regards to a even guide colour
3rd coat; for their peace of mind if they have really pale legs, a slight dusting wont really make a difference- 10 secs...
Realistically 2 coats are what you aim for. You can normally tell if you have missed, but that's very rare unless you have a colourless/guide-less solution. xoxo