You're absolutely correct. :) use "magandang tanghali" when it's around noon.
Magandang umaga = good morning
Magandang hapon = good afternoon
Magandang gabi = good evening
I don't think there's a tagalog translation for "hello".
When you want to greet someone, you can use the ones above. Or maybe say/add "kamusta?" if you see someone you know. :)