always factor in the opponent. a great return or serve is great but good is relative considering the strength of the opponent.
a number of won points is a good way to start but you don't need to serve as well to win 80% of the points against isner or raonic then you need to do to win those points against ferrer or nole.
same is true for the return, against a better server it is harder.
training all those things is great but how good it really is only shows in matches against world class players.
"Holding your serve in 80%" is not an absolute number like throwing the shot 70 feet or throw the baseball 96 but is always dependent on the opponent.
still a good serve or return does help but it is not easy to really work towards quantifiable goals.