What do you think of the idea of Simming the raid giving you your 3rd best score?
Central idea: The game keeps track of the next 3 raids' scores from each player, then only drops one from memory if the player manually plays the raid and gets better than one of those scores -- in which case the game drops the lowest and you have a new 3rd best.
It also make trying to get a better score a little more sane, since you don't have to be perfect, you just have to be better than you were a couple times ago.
Personally I see this as a little more fair than 80% of your best score, since there are a lot of people who can reproduce their past results fairly consistently, so there's way less than 20% variation for them. It also requires people to do more than play the raid once.
3rd best represents something the game can be sure you would be able to achieve if you spent the time, where at least in theory it is possible for someone to get perfect RNG on their personal best score but not be able to repeat it. So you can't get artificially boosted by having one run with perfect RNG, but you're not penalized by getting a score less than what you and CG both know you can do when you put in the time.