However, the precise resistors to be used are going to be really difficult to find.... the proposed 100M +10K resistor +10K frimer is going to yield about 5000 ohms.... to have 10k the calculator at Digikey says I need a 20.1k resistor and 20k trimmer plus the 100M resistor. That really makes it complicated. And I do understand the advantages of parallel resistors, I have gone through the great post with the teardown of resistance standards, but my needs are of the simplest design possible for calibration so not to burden some merciful EEVblog member taht may want to help with the actual calibration and also ease of construction, availability of the resistors and reasonable cost - I would reuse as much as I can what I have, the metal box, Pomona contacts and maybe the Vishay resistor of my standard (which on my 34410 meters as lower than 10k, at least after a couple hours of measuring ). Both 34410 and 34401 are still in calibration.
I guess my explanation wasn't enough obvious: Connect 100M and 10k pot in series resulting adjustable resistor from 100M to 100.1M
Now connect that parallel with 10k precision resistor and you get adjustment range from 10k||100M = 9,999k to 10k||100,1M 9,9990001k
That is actually way too small adjustment range even for super accurate resistor( I pulled the example numbers out of hat)
Lets try again: Select 10,0155k precision resistor and connect 6M to 7M variable resistance parallel to that (6M resistor + 1M trimpot)
You get trim range from 9998,8 to 10001,2 ohms.
Now your 1M trimmer 100 ppm tempco has practically zero effect on end result(100ppm trimpot tempco would cause 0,0002 ohm error or 0,02ppm)
6M resistor tempco is also largely irrelevant as 100ppm tempco would have 0.2ppm effect on the final value.
There is also some traps with this approach if you go too deep in the ppm rabbit hole..