I use a Pentair UV sterilizer, so I go by their recommendations. It seems that every UV vendor does as well. I use a Pentair 40w UV sterilizer, and the recommendations for algae control is 800-900 gph. I have a 225 gal tank. If you use it for Ick control and such then I would go down to 350 gph.
I would look up the guidelines for Pentair UV sterilizers and then try and match it to what you are using.
I also added a flow meter to it.
With a 40b, you should be fine as long as you are using soft tubing to prevent head pressure loss. I am turning my tank over roughly 4x's a hour if I want algae control.