How much overhead (%) space requires migrating a iPhoto library to a Photo library?
No overhead at all.

The Photos library created by migrating uses "hard links" that point to the actual files. This causes it effectively just point to the exact same files as the iPhoto library contained.

In my case, my iPhoto library was 40 GB. So is my Photos library. But I only had 10 GB free space. After doing the migration, I still have 10 GB free space.

(Note that you can then safely delete the iPhoto library afterwards, the file system is smart enough. See https://apple.stackexchange.com/a/182799/172153.)