Do you mean if Y becomes a horizontal asymptote as x increases? In that case you might find the limit and use the limit as an approximation, if you have to fill in numbers? Asymptotes stretch to infinity, and the slope gets incrementally smaller, so I'm not sure if you can ever really nail down a number to use for an exponent.
Sorry if I'm just talking gibberish, I haven't really done any maths in a long time.
