Bad news: this is a little harder than it probably should be.
Good news: we have an existing bug on the thing that makes it that way, so the problem is at least on our radar screen.
How I'd do this:
- Fire up TextEdit (Okay, it's a lot harder than it should be.)
- Type some text, then select it
- Use the pop-up in the text ruler @ the top of the window to set the spacing to single-spaced
- Select Format -> Text -> Copy Ruler
- Switch to OmniFocus
- Select some existing note text
- Select Format -> Text -> Paste Ruler
- Finally, select Format -> Make Default Note Style (with that same text selected)
Needless to say, I'll be upping the vote on the bug we have on the cocoa text ruler not being supported in OmniFocus. ;-)