I believe there has only been one correct answer here, at least technically. If you pay attention to the software utility, in preferences, under load, you will see something called transformer VA derating. Any transformer will do work. In the case of a MLV transformer it will turn 120v into 12v. In the process of doing that step down energy will be lost, used by or lost in the process of the transformer doing its work, turning 120v into 12v.

For the purposes of the load schedule you should just enter 50w and multiply it by the number of fixtures. But the program will add on to that number some constant, usually 20% or an other if you've entered a different value in load preferences, and make the correct load calculation. You may not see that value entered in the load schedule but try this: Try and attach 3 incandescent loads that add up to 1200w. Assuming a 20A lighting circuit try and drop a 400w MLV load on to that module and see what the software does. You will see a GRX-PB added automatically even through you have 1600w total on that module. The software is accounting behind the scenes for the transformer's energy and it knows that you have exceeded 16A and adds the power booster.

I believe the correct value, in absence of a specific transformer VA rating for the fixture being used, is 20%.