Usualy what I do (I'm not using min_z,dunno why ) is look at the possition of my model in WED(the parameters in the properties window) and then point with the mouse at his feet,this gives me the distance from the origin to the end of the model,and then in the trace I dont trace from my.z but from my.z - the value that I calculated between the .z and the end of the model. And also with use_box