Попробуйте следующее:
KeyValuePair<float, float>[] average = {
new KeyValuePair<float,float>(2,-3f),
new KeyValuePair<float,float>(3,-4f),
new KeyValuePair<float,float>(4,-5f),
new KeyValuePair<float,float>(4.5f,-6f),
new KeyValuePair<float,float>(5,0),
new KeyValuePair<float,float>(6.5f,1f),
new KeyValuePair<float,float>(7.5f,4f),
new KeyValuePair<float,float>(8.5f,5f),
};
float reputation = 123;
float review = 3.3f;
reputation += average.Where(x => review > x.Key).Select(x => (x.Value == 0) ? 0 : reputation / x.Value).FirstOrDefault();