Zooming using depth buffer

Hi All,

We are trying to implement the zoom by mouse wheel using the depth buffer in perspective projection. It is already working but we cannot find a reasonable way to estimate a minimum thresold to dive inside the model. We currently multiply current Z by 0.9 otaining a slow down very useful when you are approaching a model small detail.

How can we estimate a good minimum thresold to dive in the model before getting a speed close to zero (like 0.000000001023123) ?