Hi Murray, Nigel here.
Off the screen is the best method. IThe reference method is ideally with a measurement device like a Photo Research PR
-521, or SMPTE grade <2deg measurement device.
The best or arguably the most accurate for lumanance is the Klien K10.
All very expensive.
The new xrite i1 display probe is cost effective and really quite accurate aswell.
The traditional units was fl, at aprox 15fl off the screen
The correct si unit is cd/m2 which is aprox 51cd/m2
This is the recommendation for true theatres, many don't achieve this.
For home theatres the reality is most people tend to achieve about 10fl or 34cd/m2 with their setups, where when the bulb ages can run as low as 7fl or 24cd/m2.
This may sound alarmingly low, it isn't as bad as our eyes do adapt within reason. The trick is having total light control, your theatre achieves this. Your problem will always be because of the scope and large screens, the newer projectors are starting to become right for you now.
However because of your setup you will always struggle for high light levels when bulbs age.
The next time im up in Auckland I could point my new jeti 1211 at your gear. Oh we could do a full 3D LUT with your lumagen aswell.