I think you're a bit low. We do a 1.6 markup on fert (not sure what quantity you buy in, but we buy bulk and are able then to charge this markup). That would put you at $32 t $48 dollars per bag, just in product cost on a one bag lawn. You've then got to cover your labor as well.
Also, alot of guys charge a minimum service call charge to spread (usually an hour charge even if it only takes 15 minutes to spread).
I take my time spreading, hand sprinkle corners (i use organic), and I also go over everything twice from different angles to get an even spread. Looks way better once its kicked in.
I think you could definitely stand to charge some more.