Hi Ryan, I think that the rule of thumb used in the spreadsheet overstates the average customer tenure (as calculated by the alternate formula I outlined in another comment): https://docs.google.com/drawings/edit?id=1-9z5wczSfbM3KLZavF...

In case you find this worthwhile, I uploaded a spreadsheet showing my calculations which you can see here: https://spreadsheets.google.com/ccc?key=0Ag-jmDLo09WNdHBNY1d...