Q: How do I calculate STI with a certain signal to noise ratio SNR?

A: For calculation of STI with a certain Signal/noise ratio, calculate the SPL in your receiver point (read result in Energy parameters in Single Point Response). Then set the background noise to this level +/- your background noise SNR. Then when you calculate the Point response again, the STI is calculated with the desired background noise.
When making an auralisation with the same S/N – it is more complicated among things because the signals used for auralisation of voice and noise may not have the same ‘energy’ contents and because the playback level (SPL) on the headphone is not a simple matter to adjust to an absolute value. Probably the best way would be to actually measure the SPL from the headphone using a dummyhead or an ear coupler. As In the Auralisation chapter in the Manual “Adjusting levels there is an alternative suggestion.”

Below video shows how you can use sources in Odeon to find this total background noise level and ad the background noise level in an STI calculation.