The inlet capacity increases with the Head above it (depth of ponding water), assuming it is located in a sump.
I normally use Hydrualic Toolbox.
Now, when you have a Q and want to see if your Inlet can handle it, just enter the maximum allowable ponding depth (Head) at the inlet and determine the Q(inlet). If the Q(inlet) is greater than your Q from drainage area, you are ok. If not, resize the inlet opening. Just like Spartan5 suggested, I would follow that method.