I would suggest that you keep purity in the calculations. By that, I mean don't override the true calculated value, but rather, adjust an appropriate parameter so that the overall affect is achieved. I started with the IRO with the simple method of using default values, and then adjusting watering levels with the more/less option. Then I decided to get more technical with it, but I didn't realize, or forgot, that the more/less option was overriding my efforts. I had to go back and put all of the more/less settings back to the middle to see what is really going on, and to get accurate water usage. But it took me a long time to realize that. These overrides make it difficult for the involved user to do it right.
I like the idea of you making it easier to adjust durations and intervals directly, but I would recommend that you do it by surrepticiously having the customer adjust something like ET coefficients for interval changes, and efficiency or nozzle rates for duration.