How many friend slots do you start off with, and how many do you get per Steam level?

According to What use are Steam Trading Cards?, additional Steam levels grants extra friend slots. Given that I have as many Steam friends as there are fingers on my hand, I didn't realize there was a limit in the first place, so in the interest of finding out how useful gaining a Steam level actually is, how many slots do each level grant you?

As per this Steam forums thread, and this Community discussion:

By default, you have a limit of 250 people for your friends list and every level adds another 5 slots - which means that you get 50 slots per milestone (10, 20, etc).

In addition to that, linking a Facebook account adds another 50 slots - to a base of 300.

According to the bottommost entry of this page, the initial limit is 250 and the answer to "What do I get when I craft a badge from my Trading Cards?" on this page says that each level increases the limit by 5.