Comment 5 for bug 758236

You can see I had nearly 20GB cleared for the new install and ubiquity decided not to use it.

I tried one with a 4/10 daily where I had slightly more free space than what was available in the existing partition and it worked as expected.

I'd certainly that if someone creates a free space sufficient for Ubuntu, and they have less than 3 primary partitions, and if that truly is the "largest free space" it should be respected.