it depends on where you are in the tree and what you're opening. there will probably be a few sweet spots but in general to maximize RA output you'll want to research flat. you want to avoid having multiple lower tier techs opened, and preferably have multiple higher tier techs open. basically look at the spots where things branch out, if they're above where you are you want to finish the tech, if they're below don't finish it. as an example of sweet spot, in the current tree by completing electricity you open up refrigeration, radio, telegraph. if you have 5 techs available, one of those is going to be your median. refrigeration additionally opens up plastics/penicillin which would allow you to have 6 branches open and still get radio/telegraph as your base. an example the other way could be the wheel - it opens up horseback riding and mathematics. if you have 5 techs available, finishing the wheel in that situation will drop your median.