Which crew skills complement each other

I would have thought the answer was 72% (1-(1-.6)*(1-.3)), but all the sources I find seem to suggest that 90% is the correct reduction.

Citations: 1,2,3

This depends on the Crew Skill.

Gathering skills, (i.e. Scavenging, Slicing, Bioanalysis and Archaeology) all have nodes scattered around various planets which can be gathered from, as in other MMO's.

Mission skills (i.e. Diplomacy, Investigation, Treasure Hunting and Underworld Trading) do not. The only way to use them is with missions.

In general, even with Gathering skills, sending your spare crew members on missions will result in a much better yield of materials, along with being able to target the specific mats you need without entering zoned which you cannot handle or which are trivial to you.