The volume and tuning frequency can be adjusted with just up and down arrows. Watch the transfer magnitude function to see where the graph crosses the -3dB line. Make the graph somewhat smooth before it drops. If it has a 1dB bump it's no big deal.
To get a quick idea of how big a certain volume is, multiply the volume (if ft^3) by 1728 (12^3) to get it into cubic inches then take the cubed root to get an idea how big a cube of that size is. That's a bad set of dimensions to finalize, BTW. Try to make all of the dimensions different. This is just for a quick check.
Change from the "Transfer Magnitude" graph to "Cone Excursion" graph. In the "signal" tab, bump the wattage up until the graph crosses the red line. The red line is xMax. It's the limit before distortion.
Larger boxes will give you lower frequencies but will approach xMax with lower wattage. It's a balancing act.
Change from the Cone Excursion graph to Rear Port Air Velocity (?). At full power, ideally air velocity will be < 55 ft/s. You can change the diameter and length of the port (Ports tab) and the tuning will change accordingly. Sometimes it makes sense to have multiple smaller ports to get the right dimensions. The inside end of the port needs breathing room so it doesn't act like a smaller port. I figure that the diameter of the port is a safe distance usually.
Smaller diameter ports lower the tuning frequency as do longer ports and visa-versa.
Ideally, in the end you will account for the volume the speaker and ports remove from the cab's volume.
Modeling is only really accurate for the low end. It's not useful for the upper frequencies.
Good luck.
KO
To get a quick idea of how big a certain volume is, multiply the volume (if ft^3) by 1728 (12^3) to get it into cubic inches then take the cubed root to get an idea how big a cube of that size is. That's a bad set of dimensions to finalize, BTW. Try to make all of the dimensions different. This is just for a quick check.
Change from the "Transfer Magnitude" graph to "Cone Excursion" graph. In the "signal" tab, bump the wattage up until the graph crosses the red line. The red line is xMax. It's the limit before distortion.
Larger boxes will give you lower frequencies but will approach xMax with lower wattage. It's a balancing act.
Change from the Cone Excursion graph to Rear Port Air Velocity (?). At full power, ideally air velocity will be < 55 ft/s. You can change the diameter and length of the port (Ports tab) and the tuning will change accordingly. Sometimes it makes sense to have multiple smaller ports to get the right dimensions. The inside end of the port needs breathing room so it doesn't act like a smaller port. I figure that the diameter of the port is a safe distance usually.
Smaller diameter ports lower the tuning frequency as do longer ports and visa-versa.
Ideally, in the end you will account for the volume the speaker and ports remove from the cab's volume.
Modeling is only really accurate for the low end. It's not useful for the upper frequencies.
Good luck.
KO
how does that stuff sound? Reason I ask, I have a 12 laying around that needs to be doing something 
