How to convert RGB values to Intensity
There is no simple way to have a formula from the RGB values to intensity.
Also, there is intensity that can be measured with an intensity meter/sensor, but what people (humans, or each individual) consider more intensity (perceived intensity) is not fully related.
However, some things I found when I had a similar problem:

Typically a LED value of 0 is OFF, 1 is shining a little and 255 a lot. However, the difference in (perceived) intensity between value 1 and 10 is much higher than between e.g. 100 and 255.
Also, each LED color has a different intensity, e.g. red seems more bright than blue or green, with the same R, G or B value for the DMX lights I have, as can be read from the remarks from Duncan C and Majenko below, red is less bright than others.
Also, it depends on the physical device probably (it could have an internal algorithm to convert an RGB value to some power driven to the LED(s)).

What you can do is measure for each value and color the intensity with an intensity meter or sensor.
Or, in case you are more interested in the perceived intensity, manually check two LEDs with values to and create your own intensity algorithm.
I ended up having 3 arrays (for R, G, B), having for each value from 0-255 an 'intensity' value. I did it partly by measuring, partly by manually checking.