This is a really easy little trick.
If you want to straighten your photo, either by using an edge or a horizon line or anything in your photo, we will use the measure tool (under the eyedropper).
Just click and drag the tool along the line you want to become straight.
Let go of mouse at end of line and then go to image - rotate canvas - arbitrary, and you will notice that there is already a value set. This is the value required to make that line straight.
Hit ok and watch your image rotate.
Works every time.