Milestone leveling for a party of players who drop in and out?

Historically I have generally used XP for tracking character progress in my party. But my latest new campaign I intend to move to a milestone approach.

However the one thing I don’t fully understand is the best way to manage milestones for parties where players drop in and out of sessions. Up front we have accepted that due to life my players will not be able to make every session and so we have agreed that sessions will take place the character will just be missing, or be jaegured.

What is the best way to manage milestone leveling in this case, should I track player attendance and take this into account, level everyone at the storyline moments anyway, or take more of a player by player approach (which feels like doing XP just without the XP).

I know DnD published material is moving towards the milestone approach is there any official ruling from them with regards to league sessions etc?