This question sounds very familiar, but nevertheless here is my $0.02:
Use the listed Line Input Amperes for the Fixture or Ballast, which will reflect the computed VA Load value for a given Fixture.
This information may be found either on the Ballast's Nameplate, the Fixture's Nameplate, or from Cutsheets for the Fixture and/or a Ballast Manufacturer.
Typically, for any common Guesstimation, if the Ballast kit includes a Capacitor + is listed as "HPF" (High Power Factor), then the PF is somewhere in the 0.8 region.
A 400 Watt HID Ballast / Lamp assemblage with 80% Power Factor, should draw 4.0 Amps at 120 VAC during normal operation.
If the Ballast is CWA (Constant Wattage Autotransformer), the 4.0 Amps should remain somewhat steady from initial starting of the Lamp, throughout the normal operation; meaning:
* There should not be any high starting current demands to consider (some starting current demands may be in excess of 200% the stable operating current rating).
So, if a given Fixture draws 4.0 Amps at 120VAC, and you plan to install Three (3) Fixtures per 20 Amp Circuit, the "Subtotal" Load Current per Circuit would be 12.0 Amps.
Add 25% for LCL (unless these Lights will be on for less than 180 minutes at a time), and the resulting per-circuit designed load is 15 Amps per 20 Amp Circuit.
This conforms to the minimal NEC requests for using #12 with 20 Amp OCPD, and applying an LCL derating factor.
As always, verify that there will not be excessive Voltage Drop issues, as the HID Lamps may cycle excessively, or Ballasts may be damaged - or both.
Let me know if I did not address your question correctly, or if you have additional questions.