RPM using a quadrature encoder

I am using a quadrature encoder (CPR = 1024) to measure the rpm of a motor. For data acquisition, I use NI USB-6210. I am using one of the example VIs on LabVIEW to acquire signals from the encoder and have confirmed the physical location of the z-index. What I need help with is the way I am calculating the motor RPM. I use 'change in encoder angle per unit change in time' to give me the speed of the motor. This works fine except when the z-index resets the encoder angle to zero. How do I avoid this issue?

Thanks,

lza