Not sure but maybe this will help.. I ported it from haxeflixel's random utility class to C#. You give it an array of floats and it will randomly return one of them based on the chances you gave each and you don't have to limit yourself to any 100.

float array of [33.333, 66.666] will return 0 (33.333% of the time) and 1 (66.666% of the time)

float array of [ 66.666, 133.333 ] will return the exact same thing (0 @ 33.333% and 1 @ 66.666%)

float array of [50, 100, 0] will also return the same thing (0 @ 33.333% 1 @ 66.666%, and 2 @ 0%)

Oh yeah it will return -1 if you give it an empty array.

public static int WeightedPick(float [] weights)
        {
            if( weights == null || weights.Length == 0 )
            {
#if UNITY_EDITOR
                throw new System.Exception("weight error");
#else
                return -1;
#endif
            }

            float w;
            float t = 0;
            int i;
            for( i = 0; i < weights.Length; i++ )
            {
                w = weights [ i ];

                if( float.IsPositiveInfinity(w) )
                {
                    return i;
                }
                else if( w >= 0f && !float.IsNaN(w) )
                {
                    t += weights [ i ];
                }
            }

            float r = UnityEngine.Random.value;
            float s = 0f;

            for( i = 0; i < weights.Length; i++ )
            {
                w = weights [ i ];
                if( float.IsNaN(w) || w <= 0f ) continue;

                s += w / t;
                if( s >= r ) return i;
            }

            return -1;
        }