120 percent. A rule of thumb for line spacing (in jargon ‘leading’ – pronounced l’edding) is the font size multiplied by 120 percent, slightly greater than the capital height. For font sizes greater than 11 point or more, line spacing may increase a bit, whereas 8 or 9 point fonts can do with a little bit less line spacing. These rules of thumb suggest that single line spacing is too small and one and a half too wide.
An appropriate default setting would be 11 points (Arial) letter size and 14 points line spacing. Of course this is largely a matter of preference.