Here is my theory, somewhat same
You'll get max efficiency when the cng and air mixture ratio is at its best. Now how to judge that (the desi way):
"With max air (no blockage) start adjusting the gas intake and you'll reach a point where your car gives max RPM.. that is the point; any more gas and your average will drop, any lesser and your pick will drop. Now if for some reason screwing or unscrewing the gas adjustment nut you don't those high RPMs then it means that air is excessive so you block it to get those RPM (with min gas setting as i stated above)."
Now the answer to your main question
There are two ways to inject CNG in carb or intake system (venturi); the mechanics mostly put a small disc shape adopter on top of your air intake and attach gas pipe and put the air cleaner back. Generally in this case too much air is sucked along with the CNG, as its source is far from the venturi where vacuum is being created; so now a blockage of air is required to get optimum results.
In the second system a nozzle is drilled directly into the venturi area and gas in this case immediately gets into the intake and with less amount of air. This is a more preferred way but at times there is no space available to drill a hole then the factory fitted cars also resort to first procedure. And in a few cases where carb is made of carbon or such like material drilling hole is not recommended as it weakens its walls.