"Initial" values? What is "initial" about them? You just want to find the values of A, k1, and k2. Since those are 3 unknown values, you need three equations. If the given function exactly satisfied your given values, any three (x,y) pairs would work. If it is possible that it doesn't, the best fit would be given by using points as much separated as possible- (0, 0), (90, 0.437), and (480, 5.215).