Congratulations!!!
I wouldn't add the entire amount of CYA at once. I would start with 7 lbs for now. It is difficult to lower CYA levels and easy for there to be some error in calculation or measurement. 7 lbs will get you somewhere that will work for now and you can adjust it more precisely when you get back.
I would just pour the CYA slowly into the skimmer (after you replace the sand) and then don't backwash for a week or more.
I wouldn't add the entire amount of CYA at once. I would start with 7 lbs for now. It is difficult to lower CYA levels and easy for there to be some error in calculation or measurement. 7 lbs will get you somewhere that will work for now and you can adjust it more precisely when you get back.
I would just pour the CYA slowly into the skimmer (after you replace the sand) and then don't backwash for a week or more.