Co-channel interference

Dear colleagues,

I need to evaluate the influence of co-channel interference for 8PSK signal, depending on the relative power of signals.

There is demo example ('commadjcochanint'), but I confused.

The block, which defines the gain of interference is defined as voltage gain.

As an example, I would like to add interference with -11.2 dB relative power. It is equal to voltage 0.275.

According to this model, signals are summarized, and the max amplitude of the sum of signal and interference is equal to 1.275 (voltage).

But it is equal to 1.625 watts (1.275^2), meanwhile, the real value should be ~ 1.075 watts.

Could you, please, clarify, what do I do wrong?

Regards