OK, so then it seems the volume being pressurized is the 4X17 cylinder plus the 7.5X5 cylinder, and the pressure acts upon the entire 7.5" diameter of the nosecone. So your diameter is 7.5", and the "equivalent length" of that 17" sections is 4.8", so you could just plug in 9.8" for total length.
Also, sorry, I didn't even look to see what calculator you were using, I assumed it was
this one, but it wasn't, so my original reply didn't make complete sense.
However, using the xls calculator (or even the one you linked to) shows that 3.2g would lead to a pressure upwards of 14psi, which would lead to a force on the nosecone of close to 700lb, which is WAY more than you need to break 3 2-56 (nylon, I assume) screws. You could probably get by with 1g.
Last weekend I separated a 7.5" diameter nosecone, with about 7.5" of bay length, held on by 3 strips of masking tape (about the same as 3 nylon shear pins) with 1.1g of BP. That was, IMO, a bit of overkill, but I didn't have any redundancy and I wanted to make sure it came out. I had about 30' of nylon webbing, and rock-solid anchors, so I wasn't too worried about breaking anything. If there's any chance you might do ground testing
I'd start with closer to 1g and work from there.