Displacement by Interpenetration

A tactic often used is to have two missile units stacked one behind the other. The front one fires and then the unit behind advances causing the forward unit to be displaced behind it.

My question is, does the advance need to totally clear the unit to be displaced?

The alternative would be just to move to overlap the base a little and then have it displace.

This is voluntary interpretation by units that can freely interpenetrate each other like a pair of CLs or a pair of FLs.

The corollary is that you can not voluntarily interpenetrate a unit if you don't have the movement to do so (the rules allow S and CL to do so using their double move ability, and also allows for units to interpenetrate that start in base contact).

haven't got the rules in front of me but I think it says something like - displace if necessary / minimally displace. I'd frown heavily on someone who overlapped 2mm and displaced the unit. Maybe legal RAW but poor ethics IMO