First step is to download the XP-G2 datasheet.
Then you need to get the rated luminous flux for the flux bin of interest. For example, an XP-G2 R5 is rated at 158 lumens at 25C for 350mA drive. The key to calculate the lumens output at other drive currents is to use the graph of relative luminous flux vs. forward current. The "relative" part of relative luminous flux means the lumens output relative to the rated output at 350mA drive (i.e., 158 lumens). You'll notice that the curve crosses at 100% relative luminous flux at 350mA forward current. That means the relative luminous flux at 350mA drive is 1X that of the rated flux, which by definition is true.
At 1.0A drive, the relative luminous flux is 250%, or 2.5X that of the rated spec of 158 lumens. So the calculated output is 2.5 * 158 = 395 lumens.
You can run similar calculations for 1.2A and 1.4A drive.
These numbers are approximate since you are eyeballing the relative luminous flux off of a graph. You can get an idea of the amount of error by looking at the table of fluxes near the beginning of the datasheet. An R5 bin XP-G2 is rated at 139 lumens at 85C for 350mA drive. Using the method above, you'd calculate 2.5 * 139 = 348 lumens. But the datasheet states that the output is 338 lumens. Of course, I eyeball the relative luminous flux at 1.5A to be 3.3X, which gives me 458 lumens and that is exactly the same as what the datasheet lists.
