Spread for Life

For floaters selling at a discount/premium SFL=[100(100-P)/n + M]*[100/P] P - price of the floater n - years to mature M - quoted margin SFL is a measure of a return that accounts for the amortization of the discount/premium and constant quoted margin over the security’s remaining life. I have some questions regarding SFL. 1) Is this spread annual, for the whole life of the security or reset period specific? In literature n is indicated in years, so I guess M is also should be in years to be consistent. Then it means that the spread is an annual one? 2) Why do we magnify the spread by multiplying it by 100/P? Isn’t (100-P) already counts for the discount/premium? Thank You for your answers.