Dc level 4 18s port advice

[Coilmisfit]

Anyone out there know how much port I'd need for 2 level 4 18s....I have a bap 8 inch versa tune and a 12 inch versa tune....I'm wondering if the 8 is enough or will I need to use the 12 inch.

 Box is 11.3 cubes and I'm tuning it to 29hz, box is built and I know my lengths for ports needed just not sure if the 8 is to little or if the 12 is to big, thanks