What I do, is multiply the gauge, the stitches per 4 inches, by .72 to get the gauge of 2 strands of the yarn per 4 inches. I learned this from, I think, elann long ago, and it works very well.
Examples: baby weight at 32st x .72 = 23 st - sport
DK at 22st = 16 (15.84) st - chunky
Salt Spring Island, British Columbia, Canada