There is a simple way of calibrating your rain gauge, by measuring water amounts and then adjusting the calibration screw appropriately afterward to reflect a true reading.
The best way is simply by doing a few calculations and then making the proper adjustments to get it just right. Some of the math can be confusing to some, so you may want to get a calculator for details.
Step 1 - Calculate the Area of the Pitcher
Before checking your rain gauge for consistency, first figure out the total area of your pitcher so that you know how many inches of water is being put into your gauge. To do this you will want to measure the circumference around the pitcher, and multiply this by itself, then multiply it by the depth.
Your depth should be no more than a couple pints for this experiment. So the distance from the bottom of the pitcher to the 2 pint marker.
You now know how many inches of water you are going to have in the pitcher once it is filled to the 2 pint or 1/2 liter marker on the side of your vessel.
As an example, the circumference of the pitcher, multiplied by itself, and then multiplied by the depth or height of the vessel, will give you the number of inches in a 1/2 liter of liquid. This is your total inches, and can be calculated to the 10th's place.
Step 2 - Pour your 1/2 Liter into the Rain Gauge
Once you have your math completed, you can now add the water to your rain gauge. Be sure that every last drop gets in to ensure the most accurate of readings. Once you have emptied the entire container's 1/2 liter of water into the gauge, check and see what the result is and compare it to your inches calculations.
If it is under or over the amount that you have calculated, then you will need to adjust your gauge to show a true number.
Step 3 - Calibrating the Rain Gauge
To calibrate a rain gauge you will need a screwdriver and some patience. This is going to involve measuring over and over again an exact amount of water, in this case 1/2 liter, until you get the calibrations right. There are many different types of gauges out there and adjusting yours may be a little less or more work depending on its design.
The key here is to get a consistent reading each and every time you place the 1/2 liter of water into the gauge, adjusting as you go to make it closer to your true number.
Be careful when checking and draining your device to prevent breakages, or future malfunctions. In many cases, a rain gauge can be a very delicate device, that does not take much effort to drop and break, or even breaking the reader itself.
Do not turn the calibration screw too far in one direction or the other to prevent snapping the spring that makes it operate.