Can you post a screen shot?
Pl attach some workbook to help u
Without an example I can't be certain that this will be the solution you are looking for, but if you create a duration calc ([End_Date] - [Start_Date]), then duplicate that calc and (([End_Date] - [Start_Date])*.9). Add both reference lines and that should achieve what you are looking for (unless I am not fully understanding your problem, in that case please post example). Happy vizzing!