First, ‘repeat 6 times’ means it will water 7 times (including the first start time). Just to point this out to avoid any confusion.
Second, I would think you can set a relatively short ‘repeat every’ time to cover the cases when the watering percentage is less than 100%. Assuming you are running the latest firmware (2.1.7), it can queue the stations if the previous run isn’t completed when a new start time is encountered. Let’s say the total run time is 1 hour and you request to run every half hour, by the half hour it will queue the new run and hence they should run continuously without stopping.
There are certain restrictions here. One is that if by the time the new start time is encountered and not even the first station has finished running that station will not be queued. In other words, every station can only have one instance in the queue and cannot have more than 1 instances at the same time. But hopefully you have enough stations in the program that by the time the new start time is encountered at least the first station has finished running. Hope this makes sense.