A DMM will need to be placed in between (in series) one of your power wires going to the led to measure current at the led.
A XHP70.2 should be slightly brighter than a XHP70 at the same current. So if dedoomed you shouldn’t be to far away from what the 70 was depending on binning of both.
There is several graphs around on this forum that show how many lumens for current at the led. Up in the search box type in the particular led you wanna look at and test.