Aldrusian says:
I'm still a pretty long way before I can actually determine what a fair milestone would be.
Perhaps you are thinking on milestone leveling as too similar to XP leveling. The reason that milestone leveling is superior, in my opinion, is that you can create encounters that you want to create without worry about pacing experience points over an entire plot or adventure. This frees you to pace your adventure with a story or character arc mindset.
CR is still considered for balancing of threat to the party, but if the climax of your adventure is designed for level eight characters, then it means you can plot out all of the content you want along the way without tracking how many monsters, traps, etc there have been. You simply say, this session or this content is designed for the party at level four. This next chapter is for the party at level five, and so on. Design it all easily knowing exactly what level the party will be.
From a player standpoint, I enjoy milestone leveling over exp leveling because it's one less thing to track, and it keeps all characters in line with the challenge level of the obstacle we face. No one enjoys being too low level and feeling helpless when facing obstacles thrown at the party.