30% of the Cosmosphere is generally around the completion of Level 3 or Level 4, and you can easily get this and Finnel's Cosmosphere early on. Given the point of no return is any point after the completion of Level 7, you don't have to really worry about any issues with this other than taking the Talk Topics to get to this point, and having the Dive Points to accomplish every DP check and event.
For a complete run-down of what order of events to go through each level for Saki, refer to the 100% trophy: