Today I found a signal with SNR = 20.4. So, to not saturate the meter, better use a 21dB reading as 100%, and the better formula now is:
L = 4.76 * SNR
The bad news of this: my lnb is getting the same signal at the opposit polarization, now with SNR = 4.4. Would I say may lnb has only a 16 dB discriminating polarization factor (cant remember the exact name of this)