The LED power and brightness read by the optical sensor have no correlation, for any given situation the only way to know what suitable numbers would be is to test. You can use the IQ2 devices screen (press check to go into the advanced display), it will allow turning on/off of the LED using left arrow and also show brightness. Alternatively, print values to the brain screen.
You are right- testing is needed.
The light power illuminates the object and the sensor reads the reflection value - that was my understanding. To read a line on the field you need contrast. So if min. value on white/tan area is 80 and max. value on the black lines is 50 - setting a value of 65% would be safe to detect the black lines.