CFM depends on how much of an opening you have. If you use your hand to block the hose, you have zero CFM. For under router table DC most designs have a fresh air intake because you need to have flow. Suction doesn't move the chips, airflow does. So, if you have a 2.5" hose on your shop vac, you have about 5 square inches (4.9 actually) of opening, anything less on your table will reduce the airflow. Also, if you have too much of an opening, you will reduce the ability to pick up the chips. I like to think of it as balancing the flow though I'm sure there is some latitude. I'm wondering if the two fence ports are too much.
Most importantly, don't get too wound up about perfect DC. Give it a try and if it works for you, don't worry. If it doesn't tweak it until it works well enough.
For what it's worth, I use a 1.5 HP DC, 4" hose and a shop built Thein Baffle separator. I recently finished my router table top for PC7518 + Lift + Incra positioner. No RT cabinet yet (next project) so I built a shroud for under table DC. I left a good 6 sq inch opening in the shroud and use a fairly big insert in the lift plate. Amazingly good DC! The Incra LS doesn't have a dust port in the fence and I was concerned about that but it's really not necessary. I'll probably switch to the Incra inserts that have extra holes to improved my under table DC.