No not quite, that is a close approximation for drivers with 1 x XM-L and 2 x 18650 batteries.
It works like this: You can measure tailcap amps and the battery's voltage, and when you multiply them you get the power drawn from the battery in Watts. This is the power-in of the driver. In an ideal world, the driver would be able to supply 100% of the power-in to the led (power-out of the driver) However because of conversion loss, heat, etc. the power-out is only about 80-90% of the power-in. Now that you've calculated the power-out, you can calculate the amps flowing through the led by dividing the power-out by the led forward voltage.
So in this case I measured 0.85A over 2 x 3.9v, so power-in = 3.9*2*0.85 = 6.63W Lets assume 80% efficiency, so power-out is 5.3W. The forward voltage of the XR-E (3.7v) is higher than XM-L (3.3v), thus we can calculate the current through the led as 5.3/3.7 = 1.4A
If you take the same 5.3W and devide that by XM-L forward voltage of 3.3v, you get 1.6A, which is almost double the power-in current 