is it necessary if i use a power meter and power match? for my kickr core spindown from 24 mph only gets down to 10 mph when it tells me calibration is complete. not 0 mph.
If you are using Powermatch, it is not “necessary” to calibrate the trainer.
It may improve results, but that is not certain, so some testing may tell you if it helps in your case.
am i even getting a good spindown calibration if it is ending before it reaches zero?
Maybe. You need to see what the message says in the software you are using to calibrate.
It should indicate if it was successful or not.