The converter itself has an overhead that is relatively high when the LED is driven at low levels. There are variations from converter to converter which will effect measures. Wayne is the one to answer the impact on low with a sense resistor change on the GDx2 converter. I don't recall the relationship and dependence between low and high as they relate to the sense resistor here. Sorry.
For measuring current to the LED, I use an
Extech 30 A clamp meter. I can clamp it around the leads to the LED, zero it and then measure the current when I fire up the LED on low or high. I don't know how accurate this meter is but it seems to be quite consistent as well as repeatable. I am most interested in relative comparisons when it comes to numbers and this tool has been great on the bench! I usually just look at the current pulled by the converter itself from the bench supply, when I build the lights because this is easy to see with the head assembled and there is nominal variation from light to light. I test every light with the bench supply and this has allowed me to catch a few questionable units which by all appearance were functioning properly.