The rate of flow depends on the cross-sectional area of the narrowest part of the pipe.
The main 4" pipe has a CSA of 12.6 sq.in. If you have a 2.5" pipe, that's around 5sq.in. Your 25mm holes are about 0.8sq.in., each.
So, to balance the flow top and bottom, you'll need 6 holes, approx. That'll give you equal suck, but may not be what you want, and your requirements will change depending on the job. So, a couple more holes than that, and a shutter, ought to give you control, like a blast gate.
I'm building a table too, this year, and I intend to take the fence hose into the router box at the bottom, in one corner, with the main extract from t'other bottom corner. That should scavenge the box well, too, but it won't let me balance the suction - I'm hoping it won't be needed.
I'm also going to use Steve Msakery's idea of an extra extract hole in the router plate, for dadoing and grooving.
HTH, E.