As noted previously, the Informational Note of Sec. 220.40 is really two notes contained in one.
First, it tells you to see Informative Annex D, Examples D1(a) through D10. These walk through various calculation scenarios, illustrating how to perform the calculations in a Code-compliant manner. Time spent here to learn calculation methods is time well-spent.
If you don’t have time at the moment for such study but need to perform a particular set of calculations, find an example that can serve as a model for the calculations you are doing. You will see the correct order in which to do the work, and for many of the steps you’ll see the appropriate NEC references.
The second part refers you to Sec. 220.18(B), if your luminaires units operate at less than 100% power factor. These would be units with ballasts, transformers, or LED drivers. Unless you are using incandescent luminaires, this almost certainly applies to your lighting calculations.
Interestingly, Sec. 220.18 is right before Sec. 220.40 (there’s no 19 - 39). The rule is you base your calculations on the total amperage of the unit, not on the total watts of the lamps.
