You said 40 gpm, which I assume means flow rate (i.e. gallons per minute), not the total volume. How does one control the flow rate? Isn’t that fixed by your water source? Or did you mean to control the total volume, within, say, 30 minutes of water time? Still, I am confused how you can control both the water time and total volume at the same time — what if with your water source’s flow rate, the water time and total volume do not agree with what you want them to be set?