OPA445: CMRR: Vcm continuously changes

Part Number:OPA445

Hello,

I'm using my op amp in an application in which the common-mode voltage Vcm ( Vcm = (Vp + Vn )/2 ) is continuously changing. Since to evaluate the offset voltage Vos due the variation of Vcm I have to use the common-mode rejection ratio CMRR times ΔVcm, what is the value of ΔVcm that I should use? Should I use the variation of Vcm with respect to 0? or perhaps with respect to the initial value of Vcm?

Thanks,

Loris