With CYA of 30, chlorine of 15ppm should be fine. If you look in the chart above, cya of 30-50 we suggest 15. But, since you are in the lower end of that range and a little unsure about taking chlorine up to 15, why don't you try taking it up to 12 and keeping it there consistently for a few days. That may be high enough to take care of things. If you find that it isn't, you can always bump it up to 15 in a day or two if necessary.