for the sake of the firebox size, the 30% rule is a real close number. it can be bigger, not smaller.
for the volume of your cookchamber, the best way to do it is to figure the cylinder part of the tank and add it to the volume of the endbells. now, the endbells are the tricky part... it depends on if they are elliptical or spherical. put the 2 endbells together and figure the volume of them and then add the 2 figures together.