96 yukon timming check?

I have a 96 yukon with a 350 vortec. I am trying to check the timming with a obdcom scanner,I can see the live data and reads -23.5 deg at idel any idear if this is correct? I do know timming needs to be between -2 and+2. I wondered if this is the ecm controling it?

The spec is zero degrees and the ecm does the timing