I think you have it right, although I may be misunderstanding your explanations.
Another way to think of it is haste doubles your movement or adds 30 ft to it, whichever ends up with the smaller total speed.
Similarly, mythic haste (without the mythic power augmentation) tripples your speed or adds 50, whichever leads to the smaller total speed.
What I do not entirely understand is why this is in the third-party forum....
EDIT: I think I figured out why your explanations were confusing me. If you base speed is 10 ft, then haste doubles it to 20 ft. That is only actually adding ten to your original speed out of the possible thirty, not 20 as you phrased it.
_
glass.
