^
You need to use the actual usable space available on the card for your calcs, not the nominal card capacity.
Once formatted, a 16GB card provides right around 15GB of usable space (close but not exactly).
If you plug 15GB x 2 channels @ 24/96 into the SD calculator it will spit out 7:35:06.
Likewise, a 32GB card yields about 30GB of usable space.