The only variable that matters in the model is the size of the pay rise you get in the future. Since the proposed cap is above rpi, any rpi figure doesn't matter as you can still get above it. The only uncertainty is the level of pay rise you think you will get year after year.
You are right, no-one knows what this might be. That is the point of the modeller. Put in what you honestly think pay rises will be for the next 15yrs and the model will show you the difference if you actually achieved those rises.