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How to built a rain gauge and measure rain in millimeters

Step 1: Take a measuring jar and place it outside on a flat platform

Step 2: Measure the radius of the mouth

Step 3: Measure the volume of rain

Step 4: Plug in the volume in liters into the rain gauge - model and get the result in millimeter (you can download the model here)

Methodology

1. I take readings every day at 10.00 AM. Therefore data for Day X = Data from Day X 10.00 AM to Day (X+1) 10.00 AM. For example: Data for June 5th is the rainfall measured from June 5th 10.00 AM to June 6th 10.00 AM
2. I have used an 'almost cylindrical' measuring jar that my mother uses for baking (no more cakes at home now). Since the jar comes pre-calibrated, any error is calibration is passed on to this model. Let us assume that it is calibrated well. The cakes always tasted great - so it must be okay.
3. I have measured surface area using a standard measuring tape. The diameter of the mouth of the flask is 8 cm (+ or - 0.1 cm)
4. The flask has been placed in the balcony at a height of about 2 m from the ground. It is clear from any obstructions. I have ensured that no additional drops of rain fall into the measuring jar.
5. Evaporation losses have been ignored.

The model

To get the model, please click on the link posted below the embedded Google spreadsheet. The Google spreadsheet only displays the data. I have hidden the model which is in the first sheet of that spreadsheet. 

In the model all yellow fields are changeable. The orange field displays the result. Feel free to modify the model (area calculations) based on the specific flask that you might use.