If one of the satellites is at a distance of 20,000 km from you, what percent accuracy in the distance is required if we desire a 2-meter uncertainty

It is quite straightforward to convert an uncertainty to a percent uncertainty. We can divide the amount of uncertainty by the original amount and then multiply by 100%.

(2 m / 20,000,000 m) X 100% = 0.00001%

The percent uncertainty is 0.00001%.

The percent accuracy is the 100% - percent uncertainty.
The percent accuracy = 100% - 0.00001% = 99.99999%

The percent accuracy is 99.99999%.