Well, what's the use?
If you want to create a backup copy of the Main HD, it is necessary to have 50% each.
But if your intention is just to keep important files, you need to evaluate the formats in which they are. If documents, texts, presentations, spreadsheets and files scanned. You can use the other partition of the "security" with 35%.
But if multimedia files such as photos or videos. I advise you leave the other participation with more than 60%.
I use 70% as another drive. Ja that most programs I install are light and do not occupy much of the main participations.