Sizing the step lap rafter seat

[canopy]

I would like to make a step lap rafter seat that is as shallow a cut as permissible in the rafter. I see it is somewhat common to use a step and lap of 2" or sometimes 1.5". I was wondering how to figure if say a 1 1/4"  step and lap would be ok? The plates are 6x8 and the rafters 6x7 with an 11 in 12 roof pitch and a 5' rafter tail probably styled similar to that seen in this video:

http://www.youtube.com/watch?v=v7-kZJdLHZA

[jueston]

http://www.woodworkweb.com/woodwork-topics/wood/146-wood-strengths.html

using that chart you can get the compressive strength parallel to the grain for the plate and perpendicular to it for the rafter, after you figure out the weight on the joint of the timber and the roof, then you can divide by inches of bearing and get the PSI, check if it is within the limitations of the species your using....

but I'm no engineer.... that's just my thought....

[canopy]

I received a bit of information from an engineer and there are subtle aspects that can also be considered such as shrinkage and movement. He was saying a 1 1/2" and 2" step is a plenty for pine and historically chosen for convenience for use with a framing square. That makes sense as I see the beauty of square rule revolves around lining up an easy, accurate line with the square. In my particular case using teak hardwood it turns out a 1 1/4" step passes.