[UPDATE 1/21/20:] Some players doubt it’s an even distribution, based on feedback I’ve received. Michael Shackleford uses even distribution in his assumptions, and other players seem comfortable presuming it is. Gambling’s a tough business, with trust in short supply (just like at the Myths vs. Reality bucket on this site of things I’ve come across.) This may weigh which calculation you favor.
On the other hand, AGS’s machines have a reputation for Must Hit By progressives that largely pick values from near the top end of the range, which means you can’t do math that relies on the midpoint of the current progressive number and the top end.