Well, it's just a little bit of math. I'm assuming you're going to mix it in the 5 gallon can, so just figure out what 1/40th of 5 gallons would be. I can tell you just from memory that one quart to five gallons makes 20:1, which is what my uncle uses because he doesn't believe in synthetic oil (and barely ever rides so he can afford it!). If you want half as much oil for a 40:1 mix, then simply use half a quart of oil to five gallons of gas, and you should have 40:1.
To check and make sure this is right, let's look at it another way. One quart is equal to 1/4 of a gallon. Half a quart is 1/8 of a gallon. 1/8 of a gallon of oil, divided by five gallons makes 1/40, which is the ratio you were looking for. So we know our previous number was accurate.
Half a quart (16oz) to five gallons.