Whenever you use stencils (for helmet or armor) the first step is to do a test fit of a particular stencil with the equivalent part on the armor/helmet; it's a trial and error job. Most of my stencils are in PDF format and JPG files archived in ZIP format: the PDF file is a print-and-use type, while the JPG files can be edited in any image editor program to suit your needs (like resize, modify and so on). For example: if you are working on the left chest plate, then the first thing is to print out one of the color separation pages (not the first color page, to save your printer toner), then cut out the stencil along the outer contour and lay it on the left chest to see the differences (if any). You can resize the stencils, you can tweak it and you can cut it in multiple pieces if everything else fails; by cutting the stencils in 2-3-4 pieces, you can align each piece to the outer edges of the particular part needed to paint. You will hardly find parts that will fit perfectly the stencils, except if you use my templates for the helmet and the armor.
I hope it helps!