I would be inclined to leave all the digits there, might even help identify minor drifts with the 3458A... there are "only" 5 decimal places on the PDVS2mini, with the last one known to have a little rounding due to DAC conversion, I really wouldn't worry about it, people will only be looking up to the 5th/6th decimal place I expect, the rest is for your own records.
NIST would have trouble with 11 digits......heading into superconducting JAWS territory.
9dp's on my certs are not stable, not reproduceable so kinda pointless being there.
If i run the autocal again the numbers wouldnt be the same.
The PDVS2mini isnt that stable......so TiN is right, 7 digits will do.
It's a Precision Voltage Source, not some high grade piece if metrology gear.
Gotta be totally honest/clear when selling.........
Ian.
Update, it's fixed (example):-
volts10.Text = Format(Val(txtr1aBIG.Text), "#00.0000000")