How deep you you rank your hastati/principe?

I normally set my troops up 6-man deep. I also have my slingers / archers / velites, a long, long way out the front. As the skirmishers withdraw, I expand the heavy infantry units from 6 ranks to 3 ranks, closing the gaps between hastati. I leave the principe and triarii as is (6 ranks) until they are needed.

The problem I have with a roman 3-lined army, as that is is too deep and easily surrounded.

A question; what is the benefit of rank? Is a 3 ranked unit pushed back more easily than a 4 or 6 ranked unit?

Thanks