As for stretching PK fabric, I actually think this is the best choice for stretching and aligning!
The printed area of my artwork was quite large (45" tall x 58" wide, this includes the 4.25" black "wrap-around" for each edge) and included some straight line borders. Mine uses a 3.5" frame (incl 2" OC703 + 1.5" airgap)
I believe the larger size and gold printed "interior border" presented a worst case scenario for getting this image evenly stretched.
Sorry about the low-light photo below. However; as you can see, with gold border it may not be perfect but it is quite straight. In my case I ended up stretching it a full 1" in the vertical direction and 1.25"~1.5" in the horizontal
What I learned; if I were doing this over;
1) I could not imagine getting a straight linear result on a large size using something less stretchable. The larger and more critical, the bigger the benefit to using PK
2) As for how much to allow for an ideal stretch, perhaps half of what I did. I would propose a smaller/simple ratio like; .25" stretch for 25", .5" stretch for 50", etc....