As with any of the schedules, properly entering your crop and soil information sets the Advanced variables pretty well, certainly better than guessing. The only Advanced number up for debate is the Nozzle Inches per Hour, which unfortunately is just as important with Flex Monthly and Fixed schedules as it is for Flex Daily (although Flex Daily seems to get most of the negative press concerning this number).
If your conditions are average, with a well-installed and sized system, Rachio’s defaults for Nozzle Inches per Hour are probably very good. If you’d like to be more sure, you can do a catch cup test or measure your water usage over a given time and the area of the zone in square feet to calculate a better value. Or, you can see how the irrigation works out, and Decrease the Nozzle Inches per Hour if the zone seem dry, Increase it if the zone stays wet.