Auto calibrate varies from mfr to mfr and even between devices. However auto calibrate can be very accurate. Or if when the auto calibrate is being done you are in poor reception, then it can be way off.
I've read some devices auto calibrate several times during the ride and others only do it once near the beginning of the ride. You can, at least on my device see in the settings what the last autocalibrate came up with and if you believe it correct, then put that number in as circumference of your wheel.
An actual rollout can be very accurate or inaccurate depending on the person doing it. I put a drop of paint or something on my tire that will make a mark on clean concrete and ride my bike for a few wheel turns then measure as many marks as I can and compute the number for the wheels circumference.
That number depends on ride weight, tire pressure and other things for accuracy. But in the grand scheme.... much of the arguments between gps accuracy and wheel sensors for the average person with decent satellite reception are only arguing over a few percent. Who really cares to have that much accuracy. It's meaningless.