My main consideration for positioning the door intrusion bar was ease of getting in and out.
For the same reason, I went with a single intrusion bar so I didn't have to navigate masses of tube to enter the car.
The other issue I've had is getting everything (TIG filler rods, welding cables, angle grinder power cord, etc) tangled up in the bars as I work. Ohh, and grabbing / leaning against just-welded bits of the cage.....
It's ended up being a more substantial cage than I was originally going for. I was at a local hillclimb just before I started the cage when a 180B driver managed a double rollover, including one roll that saw him land on the other side of the tyre barriers. He was uninjured, car was a mess, roof squashed down onto the roll cage. Seeing someone I know have such a big accident made me look at it more as an important safety device than something CAMS were making me fit in order to obtain a logbook.
I've gone with the main hoop, front legs, windscreen bar and backstays in 44.45mm dia tube (up from the 38.1mm that CAMS are happy with for everything except main hoop), added an extra diagonal to the main hoop, added the diagonal braces from main hoop to front legs you can see in the pic I posted, it has a roof diagonal from driver's side of windscreen bar to passenger side of main hoop, and I will be putting a diagonal from the chassis mounting point of the passenger side backstay to the top of the driver's side stay as well as a tube between the base of the backstays (this will double as a harness mounting bar, in addition to the bars in the main hoop for wrapping the harness around).