Um, the "screw" adjustment you're speaking of is the line pressure adjustment, the flip thing is basically an on/off valve that doesn't really give you that much control.
You really need a needle valve and bubble counter, the needle valve is what's going to give you precise bubble rate adjusment, the bubble counter is what let's you see the rate of flow. But bubble counters run about $10.00, needle valves can be had for arounf the same price if your lucky. Look for a Clippard dealer in your area.