The headband does not add or subtract any bonus, so the starting value does not matter a single bit. It makes it so that your native INT is handled as a 18 when making rolls and checks.
Well, as a 17 now.
that's how it SHOULD work, but I started a new playthrough yesterday, and in dialog Intelligence rolls, it actually depicts my -1 I have because I have 8 Int, and then, weirdly, a +4 from the Headband. It's really strange, and not how it should work.