If I have a rotation that goes from 0º to 90º and I want to base the rotation on the touch of my finger, how can I calculate this?
My current situation is that if I touch at the end of the object (height equals 150 units) I would like a degree of 0º, if I touch at the very beginning (0) I would like 90º. How can put this into an equation?
From your comments, it sounds like this is the scenario.
You want to find a function $f(x)=y$ such that $f(0)=90$, $f(75)=45$ and $f(150)=0$.
Since the halfway point along $150$ corresponds with the halfway point along $90$, this is looking linear.
$$\frac{\Delta y}{\Delta x}=\frac{y_2-y_1}{x_2-x_1}=\frac{0-45}{150-75}=\frac{-45}{75}=-\frac{3}{5}$$
So that's your slope. Plug that into the equation for a line.
$$y=mx+b$$ $$y=-\frac{3}{5}x+b$$ $$90=-\frac{3}{5}(0)+b$$ $$b=90$$
Making your equation $$y=-\frac{3}{5}x+90$$