Hi,denlong wrote: This means that if all our collectors are the same size then we are exaggerating our rainfall by 9.9%.
Welcome to the forum. Fine Offset claim +/- 10% rainfall accuracy, so you appear to have a FO device that's within its specification!
But seriously, you are absolutely correct to try to calibrate your unit. However, do bear in mind that it is basically a very "cheap and cheerful" sensor, so absolute accuracy (and particularly, repeatability between different units) cannot be guaranteed (or even expected). I think the majority of people who have tested their FO rain sensors actually find it under-reads somewhat (perhaps due to splash-out). You may find this thread of interest, it's not specifically concerned with the calibration of the FO tipping bucket sensor, but it does include a number of comparative measurements made with it and a "manual" rain gauge.
You might have already found other threads on this forum (or elsewhere) about rainfall accuracy and in addition to Steve's comments above, here IMHO are a few other "important" factors: Any (tipping bucket) sensor must be mounted very firmly (not on a swaying pole!), absolutely horizontal and preferably close to ground level (but not near to anything that might screen the rain collection). Then, the FO version generally does need some modifications to reduce splash-out and to prevent the entry of spiders, etc..