The first planet you encounter, Eos is very easy to get to 100% once you complete the main mission on the planet and complete the priority mission that immediately follows to recruit the 6th follower.
The main mission you have to complete in order to setup a settlement nets 50% of the viability.
Another 30% can be gotten by completing 3 side missions which include solving the earthquake problem, destroying the Kett stronghold and discovering the research project on the planet at settlement 2.
You also get viability by setting up forward outposts, with each one netting you 2% viability for simply finding them. I believe Eos has 6-7 for another 14% viability.
In order to finish 100%ing Eos simply complete 3 other side missions that net 2% viability each getting you to 100%. Thankfully there are more than 3 side missions on the planet that give you viability making this rounding out process much easier.