I'm against finding replacements at 50+ or so. Why?
1 - There are 3 main reasons people leave parties, and a replacement can come along. First is the party has lasted a while, and the person is tired of exp. Second, either the exp sucks, or the person dislikes the party. Third, some emergency has come up. If it's the first or second reason, do you really wanna be that replacement coming in?
2 - When one person leaves, the party normally dies in less than 2 hours, often much sooner. There is no leveling spot 50+ that you can get to in under 10 minutes, and several can take up to half an hour. Do you want to be that person that spends half an hour traveling to exp for less than an hour?
3 - When most people find their own replacement, it's a rushed, half assed job to get the best person that is immediately available. You're fighting toramas, and your 62 thf has just replaced himself with a 60 drk. Your exp is gonna drop dramatically, and after spending half an hour traveling time, your party will break in shortly after the drk takes over.
What's the solution? Make sure before you leave jeuno that everyone has a few hours to exp. That way, you can get some good exp, and when someone leaves, you can go back to jeuno, and if one or more people want more exp, you can build the party back replacing the people leaving.