Right...
if the centre of your mesh is in the centre of mass of your mesh what you can do is this...
Say your mesh goes from -10 to 10 with a middle of 0.
Your fog range goes from 20 to 30
Your camera range from 0 to 40..
Why 40?
Well, your mesh will pop into view when the middle of it is within the 40 of the camera range, this means that it will extend from 30 to 50 (-10 to +10 +40).
Because your fog range is 30 your mesh will be invisible when it pops in.
if the centre of your mesh is in the centre of mass of your mesh what you can do is this...
Say your mesh goes from -10 to 10 with a middle of 0.
Your fog range goes from 20 to 30
Your camera range from 0 to 40..
Why 40?
Well, your mesh will pop into view when the middle of it is within the 40 of the camera range, this means that it will extend from 30 to 50 (-10 to +10 +40).
Because your fog range is 30 your mesh will be invisible when it pops in.