need help with box

[optical]

http://cgi.ebay.com/Ported-Recessed-Sub-Box-Enclosure-2-Alpine-Type-R-12s-/260563857690?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item3caad0f11a#ht_5534wt_911

this is the box and i plan to buy two type r 12's for it, i just want it tuned to 32-34 hz and the guy said that he will cut the 2 4 5/16" aero ports to a length i choose.

what length would i need for it to come out to the tuning i desire.

thanks