Air Stone Pumps - How many LPM are needed?

Have 2:

5 gallon bucket dwc systems

They came with 3.6 LPM pumps per bucket but they dont put out much air. Im growing white widow indoors and ill flower early if it starts getting massive so the plant wont ever get larger then 3 feet tall.

I have 2 5" round airstones

What is the rule of thumb for ratio of air output per gallon?

If i need a bigger pump, does anyone have suggestions on output capbility?

Thanks

$11.00 at Walmart, has two conenctions for two lines. Works GREAT and si CHEAP!