Understanding inductor and LED ripple current in LM3409
The Inductor ripple current should be chosen according to your application . The value will depend on the available size of your driver and the range of input voltage and output load.

In your case the input voltage (42V) and output load (LED's ) are fixed. 

Choosing a high current ripple results in a high peak current which requires a inductor with a high saturation current. Also, the core losses, which are dependent on the AC part will also be higher.

Choosing a low current ripple requires a bigger inductor. This translates to a higher resistance since you need more turns , thus resulting in higher I^2 * R losses.

So, the optimum value of the inductor needs to be optimized.
A good starting point is 30-40%. 



The luminous flux of an LED  is mostly linear, but at a certain point some of the current  just leads to heat.
Thus you need to calculate based on the LED's you plan to in order to not over stress them.