I want to try to track project progress using Earned Value Management approaches, mainly for my own education rather than out of a pressing need. The metric used to define planned value (PV) and actual work completed (Earned Value (EV)) is "% Complete" as generated by MS Project rather than a dollar value, which is OK because schedule is the main thing that I can track due to enterprise constraints I have to work within.
What I am interested in is establishing robust upper and lower control limits on the schedule, but I find that these limits get wider later in the schedule compared to earlier on if I use something simple like setting limits at +- 10% of PV. This reflects the real-life uncertainty of precisely when things will happen if they are far into the future, which I am OK with. But if I'm setting control limits I probably want those limits "tighter" the further into the future I look because the impacts of slippage are also greater.
I have considered setting arbitrary control limits (e.g. +- one reporting period) but would be happier if there was a more objective/"mathy" approach. Does anyone out there have any suggestions?