This will help with determining appropriate irrigation run times, but won't affect watering frequency.
The more/less adjustment per zone only affects the duration of watering for each time the flex schedule runs, it does not affect the watering frequency.
This is the nozzle efficiency and when increased watering time goes down, and when decreased watering time goes up. This does not affect watering frequency. if you truly just want an increased watering time, I would just use the more/less adjustment on the flex schedule for each zone. This has the least amount of side effects on other factors in flex schedules.
There are two sides to this question, applying more/less water when running, and the watering frequency.
To start simple, I recommended using one or two zones, get them dialed in, then apply to the rest of the yard, but that is just a recommendation.
To really dial-in, this article helps
To make flex schedules water more/less frequently the easiest lever (without understanding/measuring root zone depth or soil available water capacity) is probably 'Allowed Depletion' (Zone --> Advanced Settings).
To see the immediate affect, you can modify a value above and then look on the watering calendar to see our prediction for next waterings/durations.
...and to finish, if your initial settings are not correct (soil, grass type, nozzle type) than the system won't perform as well as it could with better input data. Garbage in, garbage out
[firstname.lastname@example.org] can help make sure you have the correct settings applied.
Hope this helps.