In the dim past I read about a rule of thumb based on the observation that Earth's orbital speed is near the average for all of the planets orbiting Sol and Sol's orbital speed is near the average for stars in the Milkey Way.. It goes something like this:
Each step up and down the scale ladder sees a speed change of about one order of magnitude.
So Sol should orbit the center of our galaxy at about 300 km/sec, and the Milky Way should orbit the local cluster at about 3000 km/sec. As luck would have it, the Moon orbits Earth at about 3 km/sec.
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Obviously the numbers depend on how far something is from the center of its orbit etc., so they are very inexact. But you need a range of values so that shouldn't be a problem.
I suppose you could get the orbital speeds for Mercury and Neptune or Pluto and use a similar rule of thumb to add predictions for minimum and maximum values for other scales to this average value.
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I'm still wondering if we know for sure that stars orbit (as in a closed repeating ellipse) the center of their galaxy, or if they spiral outward. After all, we've never actually seen a galaxy rotate. And accurate distance measurements are only available out to about 1000 ly. And (if you believe SR) time is a function of distance as well as speed.
LB
