When using the 8508A in 2-wire resistance mode, finding the total uncertainty can be a complex task.
To start, we need to consider the base specification. I am using Normal Mode and I am making the assumption that I am not going outside of ±1 °C from the temperature at which the 8508A was last zeroed. I am using the 1 Year specification and using the absolutely uncertainties to include the effect of the last calibration.
Next I'll review the applicable footnotes for this table:
 Specifications apply for max resolution in each function, normal mode
 Assumes 4 hour warm-up period
 Input zero or offset null required whenever the temperature moves more than ±1 °C from the temperature at which the previous null/zero was performed.
 Tru Ohms mode available on 2 Ω to 20 kΩ ranges. Read Rate reduced in Tru Ohms Mode. Specification for Tru Ohms same as corresponding Normal or Lo Current range
 The maximum display value for the Analog to Digital converter is 199 990 000 counts. This sets the maximum value measurable on each range to be a one followed by four nines. For example, the maximum measured values on the 2 V range on DC Voltage are ±1.999 900 00 V. However, the 1000 V ranges are limited to a maximum 1050 V.
None of these footnotes apply to my example, so no additional uncertainties need to be added from them. Since I have already stated that my temperature has not varied more than 1 °C, footnote 3 does not require further action.
Next I will look at the secondary specifications for Normal mode.
I am not doing transfer mode or adding anything for temp in this example so I can disregard it. I do need to make note of my measurement current because that contributes to my 2-wire adder formula.
Footnotes for the secondary specs are the same as above and like above I can ignore it all together.
Thus far I have 9% of reading plus 0.7 parts per million (ppm) of range.
Now we can look at the 2-Wire Adder formula.
I can ignore the ±3 mΩ/°C as I've stipulated that my temperature is still within 1 °C.
My Ir is 10 mA (taken from the secondary specifications table).
Here is my formula for the adder:
±(10 pA/10 mA) * 1E6 ppm of reading ± 50 mΩ
We can calculate this to be:
((10E-12 A / 10E-3 A) * 1E6 ppm) * 10 Ω + 50 mΩ, or
(0.01 ppm) * 10 Ω + 0.05 Ω = 50 mΩ
In the case of this example, the 10 pA/10 mA * 10E6 PPM of 10 Ω becomes such a small number (10E-9 Ω) that it is several orders of magnitude smaller than the fixed contribution. So assuming a reading of 10 Ω, the only additional adder you need is the 50 mΩ.
My initial spec was:
9% of reading plus 0.7 ppm of range
My 2-wire adder is 50 mΩ.
So my total specification will be:
(9% * 10 Ω) + (0.7E-6 * 20 Ω) + 0.05 Ω = (0.09*10 Ω) + (0.000007 * 20 Ω) + 0.05 Ω, or