It’s an interesting problem. How much historical data do you keep? I see two potential solutions
You could calculate the bias for a particular station if you have history
– or –
Look at a running average, Exponential or Simple.
Although the most simple might be just using a persistence forecast.
Today I switched back to Flex Daily just to see what is going on for my yard. Currently my running average over the last 6 days is about 0.065. The forecast for today is 0.11. I suspect that when the algorithm runs, September 6th will drop to that 0.06~0.07 range. This is based on what I saw over the summer. Persistence forecast is pretty accurate for 1 day forecasts.