Does that formula work out pretty well for you? That is what I thought I did once and it just ended up being way overexposed, so I figured it might be a more linear function.
It works pretty well but I must admit I do some tweaking before I get what I really want. I think it may be due to the original exposure not being as true as we might think. Any error, even 1 second, when magnified will distort proportionately. In the example above, if the 16 second exposure could have been done in 15 seconds the elevated exposure would be almost two seconds different, thus some tweaking is required.