Measuring the rain behind Skiddaw

We were pinned down by a 20 minute shower in the Skiddaw Forest. I was musing about the white noise of the random raindrop pattern on the membrane of the umbrella and then took to measuring the drop rate from the corners. I was able to count 20 drops in 15 seconds. Then the rain eased and there were 12 drops in 15 seconds. Then the rain came on heavier again so that it was impossible to count the drops. I'm wondering about trying to work out how to calibrate the drop rate to measure the strength of the rain.