How fast do you want the PCs to level? It's 10,000 XP per level so if you want them to level every three sessions on average, they should get around 3300 XP per session. Depending on your expectations you could make that your "about equal success and failure" award (or maybe "more success than failure"). They still have the opportunity to do better or worse depending on their performance.
Another way to think about this is your long-term plans for the campaign. Suppose you want to run the campaign for a year, and you expect to play every other week, and you want the characters to end up around level 15. That's 26 sessions over which you want them to earn 140,000 XP so target awarding about 5400 XP per session.
All of these numbers are totally arbitrary, it's just about matching them to your goals for the campaign.
The rate of advancement also doesn't have to be uniform. E.g. I've targeted having the characters advance every session or two for the first few levels, with a plan of then slowing down. I think it's best to lay this out to the players in advance though. Otherwise they may be unhappy if the rate of advance suddenly drops. (No one will really complain if it increases!)
Also, you might look at RMU for another goal-based XP system.