[MRCboicgy]

Here's how we work it:

If it's just a North American flight and nothing else, you don't get anything but meal expenses. If it's early flight+meeting on say, a Sunday, which does happen, then its expenses + half day in lieu. If it's a tradeshow all day Saturday then a red-eye home, you get a full day in lieu.
Overseas, there's a ton more leeway given, more to help ensure good mental health.