I have had success with a 1/8" running stitch, then a 1/4" or so parallel about an inch apart. Given the looser weave I was dealing with, it made sense. Also, remember-wool gets stretchy and shrinky, so try to use a thread that does the same (I made wool flashes once with linen thread...they didn't last but maybe 2 wettings/dryings).

Mark