Need some help with my box design (Fi Q12)

Hi everyone, first time poster on this forum. Posting all the way from Australia.

Okay, So I'm in the planning stage for a system in the new car, and I've just started doing up some rough design/placement ideas on Inventor, when I ran in to a problem. I'm designing this for a Fi Q12 ported, and I know the recommended enclosure sizes given by Fi of 1.8-2.5 cf @ 28-33Hz. However, that is quite a large margin of variation for a recommended enclosure volume (0.7 cf and 5Hz tuning frequency can affect the sound a fair bit). As I'm not a massive fan of guessing/speculating about the best enclosure volume, I decided to pull out the calculator and the old loudspeaker design cookbook to calculate the volume by hand. This is the method I have used to calculate all my enclosures in the past, and I have always been extremely pleased with the results. However, after plugging in the Fi's parameters into the formula, the enclosure volume I ended up with was around 0.7cf. Now all the ported 12'' enclosures I have built before came in at +/- 2cf (which is in line with the recommendations from Fi), so I immediately thought I had made a mistake. I recalculated it several times, and even ended up trying a few of the on-line calculators available and they all returned around the same value of 0.7cf. I have found that the main parameter causing these rewsults is the vas = 28.3litre for the Fi compared to 66litres for a JL 12W7, giving the JL just over twice the box volume of the Q.

Now, it would be brilliant if that 0.7cf enclosure volume is correct, but common sense (and Fi's Recommendation) tells me that it's not. So I'm wondering, if someone (possibly from Fi  ) in the know could shed some light on the calculations Fi could possibly have used to come up with their recommendations. I'm not really after a general recommendation/speculation based on power, type of music, etc, etc, as I can do that myself. I'm after a formula, or a precise box volume and tuning frequency backed up by some good old fashioned maths.

Thanks.