Converting tailcap amps to emitter amps in combination with a buck driver is like doing lux measurements by holding a light meter in one hand, a thrower in the other, stretching out your hands to roughly 1 meter between them and take a measurement.
Its not accurate, its not "effectively" or "theoretically" the same as doing it precisely from 10meters.
You dont know the efficiency of the buck driver so there is no way to know if its accurate. Doubling tailcap current in two cell series lights just happens to often be quite accurate on many typical budget lights driven to 2-3 amps and with freshly charged cells. But its not always the case. With a very efficient driver (like LD-29) emitter amps could even be higher.
Lets look at some examples.
Example 1:
Emitter current 3A on Noctigon. Vf is 3,4V (Vf number by djozz)
Energy at emitter =10,2W
Tailcap current 1,5A x2 cells at 4V (with load)= 12W Energy input to driver
Efficiency is 85%.
Example 2:
Efficiency is 85%
Emitter at 6A and emitter Vf at 3,9 (VF number by djozz)
Energy at emitter= 23,9W (notice that required energy at 6A is more than double of 3A due to the higher Vf)
23,9W/0,85= 28,11W input is needed if driver efficiency is 85%.
3,6A tailcap curent * 3,9045V (with load) * 2 batteries=28,11W input.
(I assumed voltage sag under load. Numbers above may not be 100% accurate, but should give a good picture.)
-1,5A tailcap current from two cells equals about 3A at the emitter with 85% efficient driver
(Simplified, saying 1,5A *2 cells= 3A emitter current is spot on in my example)
-3,6A tailcap current from two cells equals about 6A at the emitter with 85% efficient driver
(Simplified, saying 3,6A *2 cells= 7,2A emitter current is way off)
I never assume emitter amps with buck drivers, I doubt vinh does either. Especially not when he specifies emitter amps.
Always measure at the emitter at this level instead of using simplified assumptions. Efficiency of buck drivers often range from around 70-95%. And even if you assume 85% or 90% or whatever efficiency, its not as simple as to just double tailcap amps to get emitter amps. It only works in certain situations.