Do you want to soak in lower temperatures, or are you trying to save power? If the former, then set it where you like to use your spa. Then make adjustments to the ACE system as you go, just any other user.
However, if you are dropping the temp to reduce power consumption, be aware that it is generally best to just 'set it and forget it.' The tub is designed to stay at your desired temp all the time.
Also - you will get two big surprises:
1. If you have to predict your use and pre-warm the tub your usage will drop off drastically.
2. Your tub may stay at temp anyway - it takes a lot of time to cool down - and the various systems in your HotSpring tub tend to keep it warm anyway.