Done.
My unit is running fw 4.2.0. S/N begins with 2405. Verified with DC power in and without, and different backlight settings.
10v is spot on in any speed setting.
7.5 gives 0.001 error margins
2.5 gives 0.0005 error magins +/-
5V gives 0.0012 error margins +/- (observed down to 0.0007).
Using the probes that came with it.
DC power in, when present, came from a relatively clean multi-port AC USB charger I have used for years (old Anker).
All are "minus" differences (that is, the reading is that much below reference).
Tested an eBay Aidetek VC97+ DMM I had close by, with much larger error margins immediately obvious.
This is not lab testing you would be charged for... but to me it seems pretty spot on for the price range.
I hope this helps other people.