I hope I'm reading your problem right...otherwise...
Lol. It's simple math...(Ha! If only.) Just figure out how wide the chart is from left to right...then you subtract that from the total number of cast on sts. and divide it in half. For example, you have 180 cast on sts and the chart is 85 sts.
Take 180 and subtract 85...that's 95 (which is the number of sts that's not on your chart)
Take 95 and divide it by 2...that's 47.5
So knit 48 (or 47), then do the chart, then knit the last 47(or 48) (Just remember to seed stitch the first and last 9.)
So you have your first 48 (or 47)...your charted 85...and your last 47 (or 48)...add it all up and you get 180.