I’ve got a handful of goals in which I’ve set both the slope and an autoratchet. The slope enforces the minimum that I have to do, and the autoratchet enforces the maximum benefit that I can get from ‘drinking too much water’, or whatever. That may or may not be a useful framing.
At risk of confusing any actual newbees, here are some thoughts:
One of the custom aggregation methods is @drmaciver’s triangular minding, which causes each datapoint to be worth more than the previous datapoint. That’s the opposite effect, but makes me wonder whether there’s a clever mathematical formula that reduces the value of each additional litre of water in a way that’s compatible with your desire.
Should also draw your attention to the IFTTT Macros and to Zapier, in case some chain of goals makes the desired effect possible.
Failing that, for complex translations, I use our webhook callbacks and my own server running a custom dancer script.
Changing the road width is liable to confuse things further.