If a pattern is designed for only using stocking stitch, and you want it to more fancy looking by adding cables. How many extra stitches or go up how many sizes to make sure it fits properl;y? I think the cables will “shrink” the width of the piece. THanks
In my experience, recently working with adding cables to
designs, the only way I have come up with accurate figures
is to do a gauge swatch in plain stockinette and then
do a swatch with whatever cable pattern I plan to
incorporate and see what the difference in stitch count is.
It takes a bit of time but I don’t see any other way of
knowing for sure how much the cable pattern is going to
draw in your knitting. I use the cable swatch as an
opportunity to try out a few cable patterns to see which
one I liked best with the yarn I am using so it has other
uses as well! 
Libbie