4th order isobaric bandpass sub design

Quick question:

I want to design a 4th order isobaric enclosure for these subs:

http://www.manualslib.com/manual/282651/Infinity-1250w.html

And I was using this calculator:

http://www.carstereo.com/help/Articles.cfm?id=27

I want to achieve a bandpass of ~20Hz - 60Hz

I'm a bit confused though the vented enclosure comes out at 0.65 Cu ft. Can that be right? Is that even big enough to house the woofer? Seems tiny to me. Just wondering whether the calculator is just rubbish?

It's possible to have a very small calculated cabinet. Isobaric setups do just that. They make for a very small cab compared to the same speakers in for instance a straight BR or BP setup. No free meal though... what you win in less occupied space, you need to make up with much, more power to get the same SPL. In my view, isobaric designs are most at place in home environments, where as little as possible occupied space usually is a good thing to have and SPL needs are somewhat moderate and can to a certain point be met with power (mains don't run out that easy). For pro use/on the road they don't make much sense. Even for car audio I can't see any benefit apart from the size if your SPL needs aren't too big (can draw only so many amps/watts out of a 12v battery).