How much overhead (%) space requires migrating a iPhoto library to a Photo library? After migrating an iPhoto Library to a Photos App Library, they see actually share space by using hardlinks. So even the Finder says each folder use some amount of space, they are actually sharing it. That only applies if they are in the same volume. For example, both in the internal disk or both in the same partition of  the same external drive.

However, migrating could share the actual pictures but each program has specific extra structure. It could be from thumbnails, index and other cached information.

How much extra space do I need, for example, for migrating and 150Gb iPhoto Library in an external drive?

It's important to know if the iPhoto library is in volume, let's say, with 170Gb. Will be the remaining 20Gb enough for the Photo App index?

The situation is particularly important for many Macbook air users as many of them, having small SSD disk, store their pictures in an external drive.
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.)