As for the Codex nebula requirement: for the pre-Odyssey stuff, there has been plenty of work. Bark Mounds have been mentioned here before, especially as having been problematic. When it comes to Notable Stellar Phenomena that require nebulae, in the cases of large life (molluscs, pods and trees) it's the usual 100 ly from a nebula center, and for most Anomalies, it's 10 ly... except for the E02 and E03 Anomalies in Izanami, which go out to 100 ly instead. Even for large life, there are some that go beyond 100 ly, but those aren't listed as requiring nebulae in the Codex. (Their spawn zone was set manually though.) Of course, the Codex has numerous errors when it comes to the NSP requirements listed there.
For the Electricae Radialem: I don't know of any exhaustive effort. When Odyssey was new, I checked IIRC around 5-7 nebulae, the Electricae spawned within the usual 100 ly of those, so I just assumed that's universal and went on to do more interesting stuff. Looking at the data that has been gathered since (see the map
here, that assumption was probably wrong. Which is somewhat unfortunate, because it'd be more convenient to just go with the same distance everywhere than to have to do two separate sets.