RE: How do you score your shooting?
Davidmil pretty much nailed how it is in my neck of the woods as well.....10s, 8s and 5s with the occasional 12 throw in if you score it that way. More often than not in my area they count that 12 ring as an "X" so if there is a tie-breaker then the guy with the most "X's" wins.
Something else though. In my area there are three different colored "stakes" at different distances from the target. The white stake is the closest and is never past 20 yards. I believe this to be primarily for kids or beginners. The green stake is for "hunters" and is never past 35 yards...and the orange stake is for "competition shooters" and is never past 50-55 yards (IFIRC). I know this type of issue varies from area to area so just take it with a grain of salt.
To answer your first question a 298 out of 300 would be one heck of a score...at any level.