Mapantz:
I know the feeling of frustration and while I won't detail it here, one summer when we DID have rain at regular intervals and varying amounts, a retired friend set up about a half dozen tippers, and five varying sized fixed rain gauges, including two of mine which were old national weather bureau and an expensive plastic COCORAHS type along with another that was just a straight tube.
These were all mounted on top of a flat roof with their tops about as close as possible to the same height, not too near the edges so wind effect was minimized, and the roof was level on one end with a earth berm so we could walk out onto the roof and take very careful reading (from the manual ones) and dump them, check for bugs and insects, etc.
Only a few times did he ever see a rain storm register close (within a few 0.01" of each other, with the straight tube, non-multiplying manual gauge being very hard to read to any degree of accuracy being the most difficult to judge small amounts of rainfall.
Rarely did the tippers agree precisely. 0.05" was a happy day.
We did the exact calculations for the size of the collection throats, with very careful micrometer measuring to make sure we got the numbers right, and calculated the weight of the water to put in. Obviously the manuals we could just pour it in. The tippers needed to have a slow enough rate that they wouldn't drip while tipping and under report the amount of rain, which was an enormous pain and time consuming especially having done them at least three times. Those calibration checks DID show up the same or within a 0.01" per 1/2" or full inch, so we KNEW that under best conditions, they WERE measuring correctly.
The end result is that why not just bite the bullet, calculate or see what the manual says for the amount of water to use, the rate to use and weigh the grams out and do a test an afternoon or two. I used an old IV tubing with a little knob that adjusted the drip rate so it was a controlled rate, but not everyone might have access to that. And the commercial drippers are obscenely expensive for a piece of plastic. The rate doesn't have to be constant, so a small plastic cup with a small hole in it will drip through over a period of time, just don't run it very fast.
Why did he and I do this? Because we were bound and determined to have exceptionally precise measurements of the rain. Now I know better. The best example of local variation as Ocala points out can be quite different even across a few feet, was when one of the rainstorms had one of the tippers show almost 1/2" MORE than all the others. There had been a period of heavy rain, and wind, so I surmised that those sheets of rain you see falling might have swept across the one gauge, causing more to fill than those gauges even next to it. I've seen it raining heavy on one side of my house and look out the opposite side where literally there wasn't any falling, at least yet. So there are even micro-micro variations.
DO THE DRIP TEST, then you'll know how accurate your spoon tipper is and if you need to just trash it and wire in an old fashioned teeter totter type.
Dale