Way of the Wicked - Story based leveling

I'm going to start this off by saying I'm a fairly experienced player, but brand new to running the game. I chose Way of the Wicked for my first foray, as it pretty much holds your hand if you let it.

That being said, I have a 6-person party, meaning I've had to ramp up some of the combat to keep it challenging. Does anyone have any advice for doing story-based leveling as opposed to calculating the XP?

They are about to start Act 4: Burning Balentyne.

Thanks in advance!

Volume 6 has a timeline for the whole campaign, including recommended level-up points.

The AP does mention a few times that if the PCs are "behind" in terms of levels you could write sidequests for them, but I ignore that and just level up based on the timeline in book 6. In effect, this is "level up based on story" (almost like "XP based on story" but less bookkeeping.)

Incidentally, that timeline is so good I used it instead of my usual flowchart. Although of course I added notes afterward.

very beginning of all the books it has a recommended area for levels, I generally went by that as a guideline.

Thanks for the help, I didn't think to check the last book for a timeline. Once I get a better handle on building combats, I'll probably give them chances to go 'off book.' Thanks all!