When I was taught how to cut a step lap rafter seat, I was told to first layout the seat as if it was a 12/12 pitch. Then when you change the pitch to 8/12 the lines will move back away from the eave edge of the timber.

To layout a seat you use your framing square and a set of stair buttons.

Do you have any?

The picture you posted, I created.

I can create one for you at 8x12 pitch and or make a measured drawing of the distances of the of each of the cuts.

But I can't do it until the end of the week, as I am away from home on a job.

It isn't difficult once you have the button. They attach to your framing square and make it easier to layout this join.

One of the first things you need to tell us, and understand is that the thickness of the tail that laps over the plate is usually the same depth as the step.
The two most common are 2x2 or 1 1/2 x 1 1/2.
Which do you intend to use to make the 8x12 pitch?

Jim Rogers