I was thinking about this as well. Technically, if you followed the advice in the book, the characters could be WELL over-leveled by the time they met Strahd. The book gives three (non-exclusive) examples of where you could gain levels: when the party finds one of the three artifacts (like the Sunsword), when they defeat a major villain (like the hags at the windmill), or when they accomplish major story goals (examples given are: lighting the beacon, stopping the druid ceremony, and getting Ezmerelda to join the party). Here's how I could see those playing out:
- discovering the way to the basement in Death House (part of intro)
- ending Death House (part of intro)
- find Tome of Strahd
- find Sunsword
- find Holy Symbol of Ravenloft
- saving the church at Vallaki
- defeating the hags at the windmill
- lighting the beacon at the silver dragon place
- dealing with the Abbot
- defeating Baba Lysaga
- dealing with Van Richten's tower
- teaming up with Ezmerelda
- saving the winery
- stopping the druids
- stopping the werewolves
- dealing with the Amber Temple
And those are JUST the ones that make sense according to the book advice! In fact, that's a pretty strict list. Sure you could cross off one or two, but that's still well over level 10 by the time you even get to the castle. This list gets the party to level 17, and that's leaving out a lot of other places that could also easily qualify:
- getting fortunes read by Madame Eva
- rescuing Mad Mary's daughter
- dealing with Doru
- getting Ireena to come with you
- getting the "special" NPC foretold by the cards to come with you
- saving the girl on the lake
- curing the mad wizard
- freeing Vallaki from the current burgomaster
- also stopping the Watchers
- killing the head of the werewolves (could be separate from stopping the werewolves / saving the children)
- solving the puzzle of Van Richten / reuniting him with Ezmerelda / helping him with the tiger plot
- restoring the winery (as in, return the gems - separate from saving it)
- helping the lich in the Amber Temple, or saving the guy from the Amber Temple
- raising the boy from Krezk that died
- clearing out the monastery (separate from dealing with the Abbot)
- defeating the Roc at Tsolenka Pass
- defeating the giant goat at Tsolenka Pass
- stopping the witches in the castle
- finding the Icon of Ravenloft (it's a legendary item, too!)
- clearing the crypts in the castle (seriously this is worth something) and/or destroying all of Strahd's vampire spawn
- fighting Strahd in the place foretold by the cards
And of course:
- killing Strahd - at that point the game is over but it's still worth a level
Frankly, ALL of those things deserve some kind of reward in my mind. And there are more places in the castle that could qualify as well; that place could easily level the group a few times in my mind.
So the book advice could have you at level 20 - which, maybe that's a feature not a bug for some groups - but it will certainly require effort on the DM's part to upscale the challenges to keep them relevant. Or you could be much more stingy with the levels, but my point above is that even giving one level for nearly every area still gets you way over level 10.
The other option is to use the "milestone" rewards as defined in the DMG - where each "milestone" is only equal to XP worth a "hard" encounter for that level. For example: if the party is level 6, each character gets 900 XP per milestone. But 900 XP at level 6 is only 1/10th of the XP required to get to level 7! (Actually, a "hard" encounter is pretty much always worth 1/10th of a level, apparently.) That seems to go back the other way and get way too slow to me. You'd also have to include XP from monster kills this way as well, but strictly going by "it's worth a hard encounter" seems like you'd be conversely under-leveled. The best solution is probably somewhere in between, but it's hard to know what's too much and what's not enough.
In the end, I think 77IM's advice is probably the best: just use the "areas by level" chart as a guide to where the characters should be, and they'll level up at about the right pace.