Technically it's not percentage.
rand(7) = random between 0,1,2,3,4,5,6
rand(3) = random between 0,1,2
rand (1,5) = random between 1,2,3,4,5
rand function will always pick one random number depending on what you specified
So example:
if (rand(5) == 1){
mes "Number 1 was picked via rand!";
} else {
mes "Either 0, 2, 3 or 4 was picked.";
}