The rule means the average of the skills of the drill you train is maxed out at 180. As an easy example if you train gym to raise fitness and strength, this drill will become maxed out if fitness + strength = 360 (as these are the 2 skills that improve with gym)...
If you then have, lets say 180% fitness and 180% strength on your player, and you want fitness to get even higher, you must change the drill. For example if you have speed still on 40%, you could choose long run. The average of long run is then (180 + 40) / 2 = 110 and you can (assuming all the skills we talk are white skills) add another 70% on fitness with this drill, until it reaches 250% fitness and the drill will also be maxed out.
And yeah in my opinion it is also not really necessary to have 300 skills. In my experience it doesn't make a big difference if you have 200% white skills or 300% on your player.