That's indeed simpler than I though ,
The thing is that your two values are not mapped from 0 to 1, so it add a bit of complications in the expression.
I haven't tested , but the expression for Zoom should be something like :
Amount/4.8 : we take the actual amount value and his maximum and remap it from 0 to 1.
1-(Amount/4.8) : if amount = 0 we get a value of 1, if amount = 4.8 we get a value of 0
(1-(Amount/4.8))*3.5 : we remap the result so it goes from 0 to 3.5
You just need to replace Amount by the proper python code to get the value of the amount slider.
BTW, I think you can post videos as long as image in the forum, so you may not need to use turbobit.