I spoke to a man at Garmin, many years ago when Selective Availability was permanently switched on. He said that they use a 1 per second sample rate and cumulative averaging. For Velocity this applied to the track and rate of displacement. The example he gave was that if, in a boat, you tracked to a buoy at a particular position for several minutes you would hit it, even though the indicated error radius was 75 feet. The limiting factor was that the track had to be constant and, if the ETA was to be accurate, so did the rate of displacement. This, of course, hasn't helped you much with the maths but perhaps a call to Garmin UK:
(The Quadrangle
Abbey Park
Romsey
Hampshire
SO51 9AQ
United Kingdom
01794 519944)
might still get a knowledgeable and friendly response.
GBZ