Tusk Soffit joist tenon

[addicted]

Hey gang
  Ive searched the forum for quite a while and I'm still in the same place from where I started.
Soffit/tusk tenon for joist design. After going through the TF library here at home I've found articles in Timber Framing Fundamentals, A Timber Framers Workshop and the Timber Frame Joinery Design Workbook vol 1. My confusion is with the article in the workbook where the section, Engineering Formulae, states " the depth of notch on the upper side of the beam should not exceed 40% of the beam depth." This is the same article that appears in A TimberFramers Workshop. 
Can someone explain what this means? Unless I'm missing something, a 6x8 joist with a soffit tenon needs at least a tenon 4.8 inches thick. 
Does this sound right?
Thanks
Rusty

[Jim_Rogers]

What that is saying is that the depth of the block of wood your taking away from the timber with the tenon on the end can't exceed 40% of the depth of the timber.
Not the size of the tenon.
Jim Rogers

[addicted]

Thanks Jim
 That seems to contradict the soffit tenons in all the other articles I've seen. The very next article in the workbook by Ed Levin describes a soffit tenon on a 6" deep joist with a 1 1/2" thick tenon. the 4 1/2" deep block of wood removed from the top far exceeds the previous articles limit, however it passes the shear test better than the next example that moves the tenon up 1 1/2" and closer to the previous articles limit.
Maybe the 40% rule doesn't apply to this since the supported depth includes the haunch.
Rusty

[addicted]

Speaking of haunch on a soffit/ tusk tenon. What slope is usually used here? I've found one article that shows a slope of 1 in 3 1/2 and then shows a slope of 1 in 4 1/2. Any thoughts on the what slope works better  or is easier to cut/ assemble?
Rusty