LED forward voltage drops as the LED gets hotter (and the LED simultaneously becomes less efficient in terms of lumens/watt). Vf also varies from sample to sample even at the same temperature. Process improvements lead to lower forward voltage in newer production runs of the same LED. Simply measuring Vf is not be a great way to determine current.
That said, here’s match’s XM-L testing: Results: Testing XM-L, MC-E, SST-50, and XP-G emitters **Updated**
match’s XM-L direct-copper testing: Results: Direct bonded XM-L emitter to Copper
and match’s XM-L2 testing, including copper mounted: Emitter Test Results pt4: XML2, XP-E2, XP-G2, and Nichia219! (along with sinkpad tests)
djozz also did some testing which corroborates match’s results: Crash-testing a XM-L2 and a XP-G2 on copper Sinkpads
It’s important to note that all of the above testing is done with big chunky heatsinks and that some of it is done on copper. Both things make a big difference within seconds of turning the LED on.
What’s your goal once you figure out LED current?