Return Capacity compared to Supply Capacity

Hi,
I need some advice so I am informed when the pro comes out for a Man D and J calculation tomorrow.
I have about 1230 cfm of supply capacity (from capacity of ducts, without reduction for friction, etc.).
I have 675 cfm or so of Return capacity (same note).
Also Another 180 cfm of Return capacity (unused return line).

Question: is 600 cfm of return capacity enough, or should we make use of the extra 180 cfm.

All else equal, can there ever be too much return capacity (up to the supply capacity)?

I am tending towards asking that the 180 be added to the new plenum.

Please let me know.