I have a couple of issues or changes.

First off, I think I know the reason for the way the merc fans work, and perhaps you need the same logic.

So when the cars are warm and just start, you have a heat soak condition. Most times the temp will spike up to around 230 ish.
So they must have some sort of over temp limiter to where the fans kick high due to the temp.
I agree with this procedure for a couple of reasons. If you are moving faster than the 40 percent speed of the fan, you are now impeeding air flow.
I think if you add logic to set speed based on how much over set temp you are just in case, because if you hit that temp while running you don't want to step up, you want to be at max cooling right now, then your idea of starting at 10 percent, and moving up if you don't maintain is fine.

Second, a/c really doesn't need max cooling. I don't see the point of hitting max speed for a/c. The only times I have needed the fan for a/c is when I am dead stopped. if I have any movment at all, it is enough to keep it from kicking the fan for a/c in 100 degree weather.
Here I would stick to the 10 percent with cooling taking precendent for speed.
That being said, you should have a timer on it. If it gives the signal for longer than 30 seconds or so for a/c, kick it to max cooling.

I noticed the merc fans do stay running all the time once the engine is warmed up. And I think I prefer that too them kicking on and off all the time. So my vote is for min duty to maintain say a 5 degree dead band, vs the 10 degree shut off when moving.