iDirect Spreading Gain Calculation

Regarding spreading gain simply do 10 * log (spreading factor) with the answer in dB.
So if you spread the signal 256 times you will get a -24 dB spreading loss and reduce the power spectral density of your transmit carrier by -24 dB.  In the receiver you despread the signal and you are up +24 dB gain and back where you started, less some implementation margin.

Note that spreading the signal will require that you lease more satellite bandwidth and pay many times more.  If you then add more carriers, all on top of one another, you may finish up with as bad total interference as not using any spreading and just putting the unspread carriers side by side.  Intelsat, New Skies, Eutelsat etc are interested in the total aggregate interference caused by a network, not just that from one earth station uplink carrier.

Regarding Eb/No in the iDirect modem.  I suggest you contact iDirect for advice.  Modems make I and Q measurements at the centre of each symbol and from scatter in these measurements can estimate the Eb/No and BER.

If you are wanting to calculate Eb/No from link budgets try this ebno calculator

Best regards, Eric.