Hi Rune and Robert!
I finally found the normalisation factor for my nominator coefficients.
For every k pole of the filter transfer function Sk = Ak + iBk , the gain
factor is the summatory of the square modulus of each pole:
G = (A0^2 + B0^2)^2 + (A1^2 + B1^2)^2 + ... + (Ak^2 + Bk^2)^2
Of course, for even orders, the num coeffs must be also multiplicated by
the 10^(-0.05*Ap) factor before mentioned.
Thank you very much for your help!
ps: this is very well explained in the Andreas Antoniou book (page 489)