I recently put together some information to help me determine what level of reaper to run. Here's the chart as a google sheet.
https://docs.google.com/spreadsheets...it?usp=sharing
To personalize it, take a copy and fill in your own time increase expectations for each reaper skull increase. Depending on your party's damage output and survivability, as well as the individual quest in question, these numbers will vary greatly. Biggest points of variation comes from what point you might need to adjust your playstyle, what point you start experiencing more deaths, what point you have a larger chance of failure etc..
Be sure to take into account that while this is useful for determining the optimal reaper exp, it does not include regular exp and items. Use your own judgement on how valuable those are to you.
screenshot:
You can see on the other two sheets, gain per reaper skull and chart of reaper exp (simply using the reaper exp formula).
screenshots: