The actual form of the equation you are after is exponential, where the speed it converges is proportional to the distance away.

Lets say you are staring at 1 and heading towards 0 (you can always add + subtract the offset).

So in one step you have 0.9, 2 your have 0.9*0.9 etc, but what about half a step? sqrt(0.9) = 0.9 ^ 0.5. So it is 0.9 ^ (dt).