Okay, so I have a pattern that requires a triple strand of chunky weight wool to be knitted on size 19 needles. I would prefer to use a double strand of a super bulky. Now I will be doing a test swatch to see the specific answer to this problem. But I am wondering if there is a formula to know what size needles to use when you double or triple yarn? I know you don't double the needle size. Is there conventional wisdom on this?