Autocad and drafting aren't carpentry.

It's much easier to draw it up with a computer than it is to put it up in reality.

For example, let's say you got a 8/12 roof with a an 8ft ceiling. The outside wall is staggered in 1 ft 9 inches, but the overhang of the roof is unstaggered.

I'm going to have to raise the height of the staggered in portion of the wall by a certain amount of inches so that I can rest my rafters on it. (these types of things are almost always missing on the prints, so we basically have to figure it out on the job)

To describe the algebra involved, we are moving in 1 foot 9 inches or 21 inches inward (which relates to the 12 portion of the 8/12 pitch). X will equal the height I need to raise the inner portion of the staggered wall to set the rafters on and that relates to the 8 of the 8/12 pitch.

So the known numbers are 21 inches, 12 inches, and 8 inches. The unknown is X

To find my ratio for this I divide take 21/12 * 8 = X

21/12 = 1 and 9/12 = 1 and 3/4. 1 and 3/4 * 8 = 14 inches.

Then I have to raise the bird's mouth on the rafters for this portion of the roof to suit the raised walls, thus keeping my overhang even throughout while staggering the inside wall.

It gets

*really* complicated if the rafters are Jack rafters and not common rafters.