If you're doing it for 2 subs then you need the line area to equal the sum of
Sd (cone area). In this case, 45 sq in times 2
equals 90 sq in of line area. That is, of course, unless you were talking about 2 seperate boxes.
Also, where did you get 7.5ft? For 28-35hz I get between 8 and 10ft of line. Not that 6 inches is going to kill your project, but better to be as close as you can with the math since getting it as close as you can with wood is hard enough.
P.S.- what's wrong with your little one? I hope it's nothing serious.