Your reloading right?
Here's some info with a 20" barrel I would guess you'll lose 80 to 100 FPS with a 18. Drop by a big sporting good store like a sportsmans and look at a few reloading books in the 308 pages real quick. You have months to get it all figured out and I am sure you will.
http://www.accuratereloading.com/sscout.html