I would have to disagree. Yes you do want to allow for the extra space when the pumps are off. But you dont want to mark the max line when the pumps are off.
Step 1) Turn pumps off
Step 2) Let all the water syphon down and equalize
Step 3) Fill up to a mark where you are comfortable say about an inch from the top of the sump
Step 4) TURN THE PUMPS BACK ON
Step 5) Once the water is back to being stable in both the sump and main tank, mark the MAX line
You do this because you add water while the tank is in opperation. If you make the mark when the pumps are off, then fill it to that line when the pumps are on, you run the risk of flooding your floor if power goes out.
Brian