Optimum Sealed Enclosure Volume

Enclosure calculators and software usually produce generalized volume numbers for the parameters you input.   The calculator you linked to will give you a predicted response curve.  Play with the numbers until it draws a curve you like and start there.  But will it be optimum?  No not necessarily.  Indeed, a 2.26 CUFT sealed enclosure for a single 12" woofer is quite large.  I'd cut that to closer to 1.4 cuft if it was me,.  And I always think a sealed enclosure sounds better with polyfill.  Not only that. but using polyfill allows you to tweak the performance a bit after you build.  Polyfill can increase the effective size by as much as 10%.  

But the real anser to your question is you have to build it and see.  There is no such thing as a "perfect" enclosure size or shape as it is always different for every individual loudspeaker (even two of the same model) and for every installation.  Be ready to modify, modify, modify and stuff, unstuff, stuff again to tweak whatever enclosure you build to get it right.  Have lots of bracing, polyfill, screws and RTV on hand!