OK, so the opening is 6" diameter, that makes the area of the opening pi * (3)2 = 3.142 * 9 = 28.278 sq inches.
If one inch of rain falls then 28.278 cubic inches of water would pass through the gauge.
28.278 cubic inches = 15.7 fluid ounces (US) or 463.39 millilitres (ml).
So if you measure that amount of water and drip it very slowly through the gauge, it should register one inch of rain. For a more accurate result drip a multiple of that amount of water, 2 or 3 inches, but be careful not to exceed the maximum rain rate that the gauge can handle.
