Update how-it-works.md

docs/how-it-works.md

@@ -7,7 +7,7 @@ The below image shows a graphical representation of what this integration does.
 1. Snow and rain fall on the ground add moisture. Together, this makes up the `precipitation`.
 2. Sunshine, temperature, wind speed, place on earth and other factors influence the amount of moisture lost from the ground(`evapotranspiration`).
 3. The difference between `precipitation` and `evapotranspiration` is the `delta` or `netto precipitation`: negative values mean more moisture is lost than gets added by rain/snow, while positive values mean more moisture is added by rain/snow than what evaporates.
-4. At some point in the day (configurable) the `netto precipitation` is added/substracted from the `bucket,` which starts as empty. Also, the `drainage rate` is taking into the account (if set). If the `bucket` is below zero, irrigation is required.
+4. At some point in the day (configurable) the `netto precipitation` is added/substracted from the `bucket,` which starts as empty. Also, the `drainage rate` is taken into the account (if set). The bucket is calculated using this formula: `old_bucket + netto precipitation - drainage rate`. If the `bucket` is below zero, irrigation is required.
 5. Irrigation should be run for `sensor.smart_irrigation_[zone_name]`, which is 0 if `bucket >=0`. Afterwards, the `bucket` needs to be reset (using [`reset_bucket` service](services)). It's up to the user of the integration to build the automation for this final step. See [Example automation](example-automations) for automations that people have built.
 
 ## Weekly behavior example