maybe you should look at a program to determine all of this...
http://www.linearteam.dk/
to determine what amount of air space you need, you need to figure out how much air space the sub takes up. From there, you need to determine the sub enclosure volume for a normally mounted sub, then subtract the space that the sub takes up.
In all reality, on most subs, the type Rs included, this whole process is neglegable. If you had some beefy sub (think SPL sub) then it would be a different story...
the thing is, while you subtract the amount of air space that the basket takes up...you need to add back the space that's added by the volume of the cone...
so in all reality...for most enclosures, we're talking about very small differences.