When you measure the surface of the bucket that precise, I think you should also account for the corner radii.

The faster you pour in the water from your container, the more inaccurate will be the tip counter, because you'll get splashing inside the housing. That is why your tip count was so different on the second pour. There is another video on KZfaq that shows this problem very clearly. For the most accurate measurement, you need to pour the water in at the rate of the rain.

CC = ML You made this way too hard. Get a 100 cc syringe, slowly pour water into a bucket until it tilts. The stand must be level. Subtract volume used for 1 tilt from total volume in syringe, then you have the volume/tilt. These units are already designed to tilt for a specific volume of water. Many are set for 1 tilt/.01 inches and some sre ste to .011, the extra to account for a small amount which does not fall from the tilt, of course, use a cc/mm to inches conversion as needed. You should be able to find specs on your tilt unit. Based on what I think you were seeing, my guess is 1 tilt = half an inch but use my method to measure the tilt volume. Good luck !!