To be clear, the only thing a rain sensor will give is a fail safe to NOT run a schedule if rain has fallen in your area hat was not predicted.
I have one and it is because my micro climate is a bear, we will get rain here that is forecasted, so it delays my schedule,,I have an ifttt recipe,to,set a manual rain delay for 24 hours to allow rachio to record the rain fall and recompile my moisture.
Only if you are willing to put in some work by ordering catch cups and running so,e tests. We the community can help you with setting up the rest.
They will not know. If you want accurate accounting, you have to get a pws.
Your best option, if you stay in fixed schedules, is to adjust your rain skip on that schedule to something lighter, like 1/4. But you still fish running on a day it rains with such a volatile microclimate.