I usually use a margin of 1 to 1 1/2 inches, on the right, but I write small. In conjunction with what smejs said about margins on the top and bottom, I've found that the pages where you really, really want a large top margin is the first page of the script, and if there is one, the first page of the second act. You usually end up with more cues when you're starting out sections. Likewise, you'll want a larger bottom margin on the last pages of section. Just what I've found.