I'd just send the first three chapters. If she doesn't ask to read more, then she's not your agent. Agents want to see capable writing and a strong hook right away. If those aren't present in your first three chapters, they won't ask for a full. If only one of those things is present and the idea presented in the query is strong, they might ask for a full anyway. I think that's the lesson from popupsubmissions. If you haven't got a hook right off the bat, they aren't going to read further anyway. I wouldn't worry about sending exactly fifty pages. And if I felt like those twenty other pages are so good they need to be submitted, I'd have to ask myself if I was starting the story in the right place.