That is quite a massive tank!
It looks like you are trying to directly compare the lumen output of a metal Hali lamp with the lumen output of LEDs. Traditional lamps, including metal halide, distribute their light ove a much steradian, thus the need for large reflectors to collect all that light.
LEDs distribute their light much more dictional and with appropriate TIR optics are able to direct their light output with much higher optical efficiency. So light output per steradian - light density - is more important than overall lumen output.
yep,really large tank I built myself
yes,I want equal to or greater than the light output of the halides with LEDs...I understand that the LEDs are better focused therefor waste less of their usable illumination,produce much less heat and much cheaper on the monthly power bill
problem is,I don't have a way of measuring lighting PAR to accurately know how many LEDs I will need,and I really don't want to invest in a $600 light meter that I'd likely never use again LOL so I'm kinda copying from other large reef tank owners,I at least have an idea where to start....
my thought is to remove 1 or 2 halides and build an LED array that looks close to what the remaining halides output is then turn them down,I know that's not a great way to do things LOL
I'm really happy with the color temperature and light output of the halides that I'm using now and I'm not going to LEDs because I think they will grow coral better or are a better source of light,I've got to reduce my power consumption or get rid of my tank LOL
thank you all for the replies and great suggestions,it's all really appreciated