At idle, the throttle plate is closed and the engine pumps just to get enough air to survive. This pumping generates a vacuum inside the manifold. Typically we measure vacuum in inches of mercury or in Hg. Most cars idle near 20 Hg. For engineers, we like to deal in kilopascals (kpa) since it's metric and actually makes sense. One atmosphere, or 14.7 psi, is 101.4 kpa on the absolute scale. You can find the conversion here: www.engineeringtoolbox.com/vacuum-converter-d_460.html. So 20 in Hg gauge vacuum is about 28.5 kpa.
Last time, I ball parked my calculation strictly based on the approximate percentage of pressure the vacuum accounted for and just multiplied the WOT flow rate by the percentage of the vacuum. Since pressure and overall vacuum isn't a linear relationship, I assumed a generous 33 percent of the WOT flow rate to get the idle flow rate, which brings us to about 259.3 liters per minute of air at idle.
In reality we also have to account for the volumetric efficiency (VE) at idle, now that you want to nit pick. While valvetrain and flow characteristics do little to affect the minimal flow at idle, frictional drag inherent to each engine design does. A typical VE at idle is ball parked at 56 percent, although different designs might vary by up to 20 percent. So taking our WOT flow of 1,000 liters per minute and multiplying by the VE and percentage vacuum (28.5 divided by 101.4) will get us 157 liters of air ingested per minute. Almost half of what my original paper napkin approximation was since I ignored VE. Still it's a huge amount and few people realize just how much air an engine will eat up.
So now we know how much air this engine consumes per minute at idle. This is a volumetric flow rate, not to be confused with a MASS FLOW RATE! We can't just multiply the air/fuel ratio by the volume of air consumed and figure out how much gas you consume, like you did. Before figuring out how much fuel you burn, the volumetric flow rate has to be changed into a mass flow rate since a gram of gas takes a lot more space than a gram of liquid fuel.
This requires some knowledge of chemistry and the ideal gas law, which I'm going to assume you know nothing about based on your email. You're just going to have to trust me when I say that every liter of air at standard temperature and pressure (STP) weighs 1.29 grams. For the volumetric flow rate of 259 liters per minute (68 gallons per minute) of air, the ideal gas relationship works out to be about 335 grams per minute (0.74 pound per minute) of air. With the more accurate flow rate of 157 liters per minute (41 gallons per minute), the mass flow rate works out to be 203 grams per minute (0.45 pound per minute) of air.
Now we can finally use our air/fuel MASS RATIO of 33.43:1. Take our new more accurate mass flow rate of 203 grams per minute of air and divide it by the air/fuel MASS RATIO of 33.43 units mass of air to 1 unit mass of hydrogen gas, and you have your fuel MASS flow rate of 6.07 grams per minute of hydrogen gas. Now think about this. How much volume does 6.07 grams of hydrogen take up? That's a pretty big f*cking balloon right? Again, we know through the ideal gas law that every gram of hydrogen gas at STP takes up 11.2 liters of space. That means at idle, this little 2,000cc engine will eat up 68 liters of hydrogen gas every minute, the volume 6.07 grams of hydrogen gas takes up. It's called a MASS (not volumetric) RATIO and that's how I get the volumetric flow rate of hydrogen gas per minute. That's a lot of soda bottles per minute!