krasimir.kSince you have two calibrations, what was calculated drift of your LTZ1000 standard?
The Fluke 7009N itself was calibrated in BIPM France in 20.12.2017.
Wait here a second, so standard was never calibrated since end of 2017? I didn't expect this from NMI lab, to be honest.
I'd say going for 100K modification was a good choice, I can only wish to see more people who do not need full +50°C ambient functionality ability do it. 1.5 months for settling is usually enough for LTZ1000A A9 reference. You do have to keep 3458A permanently powered on, however.
Could be best if BIM would adjust your 3458A for you vs JJ, so you could know internal LTZ1000A A9 value with very good confidence. Then you could "transport" that value to your LTZ1000 reference at home, using 3458A as a transfer box (I know, many in EU will throw tomatoes at me on this suggestion, but it's better than nothing).
Perhaps next step would be to evaluate thermal stability of your 3458A (you can cycle on/off airconditioner in the room, while logging data over few days), so you can estimate TC error of your 10V measurements as well.
Looking forward to hear more about your results.