Note the level of the rain by viewing the base of the meniscus. The meniscus is the curved surface of the water that forms by surface tension as water fills the measuring tube.
Note the closest marking to the base of the meniscus on the side of the gauge. There are typically 100 markings on the tube.
Find the rain level measurement by noting the number that the closest marking represents. A full gauge holds one inch of rain. Therefore, each marking represents .01 inches.