I understand that in doing a dual angle layout for a symetrical ball that the first angle means nothing as in 50x4x30 the fifty doesn't matter. If this is true, how does one determine the length before the ball starts to roll?

At times I wondered this myself.  However, I dont over think it.  So I try to keep it simple and think of it as pin to PAP determines when you want the core to get started and the VAL angle determines hook shape.  But bear in mind this also depends on core numbers.  As an example, unless you are speed dominate; you wouldnt put a 4 inch pin to PAP on a ball with the RG around 2.46 unless you want the ball to roll at your feet.  The exact opposite can be said with a high RG ball.  Unless you are rev dominate; you are not doing a 6 inch pin to PAP on a ball with an RG of 2.58. 

So with that said, and keeping it simple.  If I want something to roll sooner, I get a lower RG ball and match the surface of the cover to get the desired roll and backend.  If I want something more skid/flip; I go with something higher RG to delay when the core wants to roll and adjust the cover if needed for desired length and energy retention.