I don't understand what the problem is here. If you can't snap to a vertex use the grid? Move things by specific number of units? I've never had an issue with this.
you can
but the math needed to make sure that slopes and angles are perfect takes a long time to do and not everybody understands it
Dear goodness no. Why would you do math to calculate angles when Blender can do it for you? What's wrong with just rotating a plane by a specified number of degrees? You don't need to calculate anything if you know the angle?
What are you people trying to do? Describe your problem and I will be post a video showing how to do it easily.