I'm working on a project where part of the curtain wall has a few random horizontal elements that project from the vision panels from floor to ceiling.

These elements provide shading and therefor contribute to the reduction of the SHGC for the glazing.

My problem is to calculate the Projection Factor (PF) per ASHRAE's instructions (pg 28 of ASHRAE 90.1 2007):
Os = (Ai ⋅ Oi) + (Af ⋅ Of)
where
Os = percent opacity of the shading device

Ai = percent of the area of the shading device that
is a partially opaque infill

Oi = percent opacity of the infill—for glass Oi =
(100% − Ts), where Ts is the solar transmittance
as determined in accordance with
NFRC 300; for perforated or decorative
metal panels Oi = percentage of solid material

Af = percent of the area of the shading device that
represents the framing members

Of = percent opacity of the framing members; if
solid, then 100%

My English is pretty good but I don't understand this description of the equation.

Does anyone have a simpler or even a graphic representation of how to calculate the "Projection Factor"?