BLUF: How to write performance goals when staff migrates through multiple project areas in a year?

Details:  My team is a service center (aka matrix pool).  Over the course of the year we work for six project areas.  The staffing model is extremely flexible.  We gravitate through one to many project areas, multiple times, and for a variety of specific engagements. How do I write an annual performance goal that is flexible enough to accommodate the uncertainty of assignments, yet specific enough to be measurable and time-based  Also, I need 4 levels of success criteria to differentiate from "just getting the job done" to exceeding every expectation.  My first crack at it was to use level of autonomy to differentiate between the levels.  That is difficult to measure, so I'm interested in other ideas.  I posted in the LinkedIn forum as well, apologies to those of you I hit twice.