Usually, the coolant overflow tank on your car will be marked with correct fill levels. If not, I would consult the owner's manual for certainty. If you lost the manual, this Edmunds site has a owner's manual page where you can download most recent ones. Go to http://www.edmunds.com/ownership/howto/
articles/120189/article.html and you can look it up. Now if the car has not had the coolant changed in the last 3 years, I would go ahead and do the procedure the dealer suggested as you need a coolant replacement at this point anyway.