I've had our 16 zone gen 2 for several months, and with some helpful (and appreciated!) advice from a few folks here, wound up setting up a Flexible Daily schedule. My lawn is mowed on Thursdays, so I did NOT want it wet then -- so I manually selected the other 6 days for "Will Water". I set "Start After" to 5am, and turned Smart Cycle on. I had already configured the zones (soil, plants, slope, etc.) when I first set up the device, and the only other changes I've made since then have been to experiment with the choice of different weather stations in my area.
Net: the system works exactly as I had hope: the flexible schedule only waters when my lawn needs it, only on one of the six allowed days, and whenever it does happen, watering starts at 5am. That has worked beautifully for me.
As I re-read your note -- I was probably too quick to fire off the comment above. Looks like you want the system to determine what time of day is optimal for watering within the allowed time window. I'm guessing that there will be little difference in irrigation benefit due to choice of time-of-day; if there IS a difference, e.g., temperature is more conducive somehow to water absorption at midnight vs. 10pm... I'm guessing this will pretty much be a constant for the vast majority of situations. If I were trying to do this, I'd do some local research, set the flexible schedule as above, and then just pick a single "optimal" time of day for the program to always start. I realize now that's what you were probably hoping to avoid -- sorry if my answer was overly simplistic. But if there IS a significant difference in watering success depending on specific hour during your approved 18 hour window, I'd love to learn more about that from anyone who can shed some light on the topic!