Not only do I use markers, but I always carry a pair of tying sissors with me to trim flies down in size if needed. I have had it happen many times when finicky fish key into a size or two smaller than I am carrying. The hook size doesnt seem to matter , the silhoutte does.
Which brings me to the next point: Using markers and trimming flys can be great in a pinch, but really isnt or shouldnt be a substitute for tying to match a hatch.I equate it to using an adams or a bivisable.......
Most times they work ......in a pinch......Ed