Why? Why not divide by two? I think that's where most people would assume you were going with this,

I.e. a fraction equally near to 1/3 as to one whole.

I.e. the mean average (a traditional compromise) between the two numbers. (Between 1/3 and 1, i.e., roughly 33.33% and 100%.)

I.e. 2/3, or roughly 66.66%.

You seem to be proposing a weighted average, in which the smaller number is given more weight or importance, so that the compromise number will be nearer to it than to the higher fraction. Does it need to be that complicated, for any reason?