If you are talking about the piggyback guide, yes I'm using it.
Yes, you can fall behind their timeline in the 'walkthrough' section. They really haven't allowed for any margin of error. I'm sure its an optimal playthrough but it is very restrictive and you can easily fall behind just by going down the wrong street and having to turn back or forgetting to pick up something and having to go back for it.
I suggest you ignore the 'walkthrough' and focus on the 'Main Quests' section instead. This will give you the details you need on when to turn up to a specific place etc without giving you that sinking feeling of falling behind.
Just remember that the game was designed around working at an equel pace through all the main quest lines instead of just focusing on one at a time.
From what I gather, there is more than enough time to do all the main quests and still have a few days to spare to finish up side quests.