On my '04 coupe, the spoiler goes up at 58 and down at 38. You only have to cross 58 for a fraction of a second and its up and it stays up until you slow below 38 or push the switch to manually lower it. In my previous job, I had to drive 15 miles on a twisty back country road to get to work. Slowing and speeding up on that road with the spoiler controlled automatically made it look like a flapping wing. I got tired of listening to it and started manually raising it as I left.

 

My wing goes up sooner if I have the throttle floored, than if I was accelerating slowly. Makes a difference of upper 50's to lower 60's. Don't think it matters much exactly what mph it goes up at, I wouldn't think, so long as it does.
Something I noticed with my xf, is there is an extra "push" when I push the throttle all the way to the floor. It goes smoothly until the very last , oh I don't know, maybe 1/2", then it's almost like I have to push harder to get past that spot to get it to the floor. Has anyone else experienced that?
That's probably the point of FORGET GOOD GAS MILEAGE, LOL!

 

Mine goes up at 110 km/h (68 mph) and down at 60 km/h (37 mph). I didn't experienced any difference reaching the mark slowly or fast. But will keep an eye on it ...

@RickB, when I drive on similar routes you described I push the button and the wing stays up all the time (no flapping). But don't forget to deploy it manually after.