I'd find a good value and just leave it. An average should work just fine.
Note that current flex schedules DO NOT use the crop coefficient on Advanced zone settings. Still trying to catch up, hoping to start using that value sometime next week in flex schedules. I really don't want to use it in existing flex schedules since it might have a big impact on users that have their schedules dialed in, but will have it on any new flex schedules. It is probably the single easiest value to modify to change watering frequency throughout the year.
For current flex schedules we do use a a range of values each month per turf type. Here is the list of values, which I like I said will be replaced for flex with just using the crop coefficient in the zone Advanced settings.
Cool season grass:
Warm season grass:
Here is a good range of values:
Here are some examples of values the EPA uses for their testing: