Valley rafter installation height?

[Digmixfill]

Merry Christmas everyone.

 

I have two roofs at the same pitch and height that join with a pair of valleys that will have valley tiles. Common and jack rafters are 44mm wide, and valley rafters are doubled up.

 

I'm looking at valley rafter installations, and I see some valley rafters set at the same height as the commons and jacks. I see some where the jacks are slightly higher than the valley rafter, so I assume the valley rafter is set lower than the commons in this case? What I have failed to find is any information giving an indication why either variant is used. Is there a reasoning behind the choice?

 

 

[Oz07]

It's just because the valleys are on a lower pitch so the plumb cut is higher on the main plane of the roof.

[Digmixfill]

My common rafters are at 35 degrees, and I expect my valley to be somewhere near 26 degrees.  Are you saying that if I cut the heel height of the bird's mouth the same on the commons and the valley rafter, the valley will naturally be lower than the main plane of the roof?