I found the reason why I was getting greater and better multipliers.
I took for the factor k a term  log (k), but since q(x) = (sqrt(n) + x)^2  kn grows with O(sqrt(k*n)*x), a factor  log(k)/2 is more suitable.
So even that my approximation on the resulting hit rate (or the resulting length) of the factors in the factorbase is better then in other approximations, the resulting multipliers are mostly the same then in the other approximations like the implementation of alpertron :(
