You can find a complete description of Zimmerman’s algorithm on the support document page, specifically this page:
https://opensprinkler.freshdesk.com/solution/categories/5000022938/folders/5000099525/articles/5000017312-using-weather-adjustments
1) Zimmerman’s algorithm computes a watering percentage — a scaling factor that is multiplied with the user-set program water time. So it adjusts the duration and not frequency.
2) The baseline is set by the user — the user decides how long to water assuming average weather condition.