Hi fleemo,

Trial and error work best for me. If plants look thirsty bump it up a little. Irrigation duration and number of times one irrigates can do the same thing. JMHO. Aloha

Get and use a moisture meter.

It takes the same amount of water to wet the soil down to "X" inches no matter what time of year. The variable is how soon dies the dirt dry out. It's more practical to adjust the days between watering instead of adjusting the amount given.

This can't be correctly answered without a slew of info. ( type of application, soil, slope, sun, etc.) If you truly interested in setting the controller correctly, get an IA certified contractor out for one visit, have hime set the controller correctly, write it down and then make your adjustments using the Water Budget. If it's not done right originally, any adj's aren't going to help.