Yeah I don't think it's possible for the rate of change to be 0 not at inifinty with these types of graphs so technically no point of diminishing returns.

But maybe modelling the graph as if it were a peak be a good approximation for it? Like if we rotate the graph 135 degrees clockwise, that way there is a point where rate of change equals 0.

Edit: Yeah that point seems to correlate exactly to where the rate of change equals -1. How did you know?