The stones are given points (normally between 0 and 4) depending on how close to the task (as the judges see it) the stone is. This often does not include secondary tasks that the team my set up for a stone, if it is not perfect. So while in theory a game should have the same score independent on who runs the scoring, there are quite a few "grey areas" (eg how good is a guard, should it have 2 of 3 points).
There is also a system of bonus points, but I don't know exactly how these are calculated.
Divide the total points with 4 times the number of stones, and you have a percentage.