Greetings.  I have a couple of questions regarding my Window install that I will attempt in the spring, if I may?
Attached is a drawing my window vendor came up with.  It’s a first rendition to give me an idea of how to mount my windows.
I’ll slope the sill a few degrees and use shims for proper drainage.  It appears that I will need the metal pan flashing that slopes away from the window over the sill.
The sheathing which is sandwiched between the rigid and dense pak in the drawing will be my WRB.   This sheathing is Pacific-Georgia’s flavor of ZIP called Forcefield. 
I already have some SIGA Fentrim 230 15/60 tape and also some 12″ Vycor tape.   Does it make sense to tape siga from the sheathing to the buck and then from the buck to the subsill extension, or use the Vycor from the exterior of the sheathing under and around the buck, then under the subsill and to the back of the subsill?  
As for the metal pan flashing, does a better method exist rather than screwing the metal pan flashing to the subsill?   I won’t see the screws much, but I don’t like the idea of having them exposed.