The other point this obscures is that it pays scant attention to wind profile. Specifically, if you're looking at 50 mph transverse winds you aren't going to get to where you want with a super big envelope/frame. Drag is the cube of airspeed, so the bigger airships are, the less control you have over where they go (without adding lots and lots of power to the power plant).

Try off-loading containers when you're "ship" turns 90 degrees in < 30 minutes because a breeze came up.