How Fat is too Fat? How Thin is too Thin?

Apropos a conversation with Etheline this very morning I have a few questions.
At 5 '10 according to most experts and BMI charts ( I know, they're only an estimation) I can carry anything between 140 pounds to a 180 pounds without being considered particularly fat, and that's not taking muscle into account.
I weigh, as of ten minutes ago, ( I had to check) 150 pounds. Personally at 180 I would be extremely unhappy and uncomfortable, I would carry most of that weight on my face and my stomach. I would definitely be, in my head, overweight. No, I would consider myself fat.
At 5' 9' Etheline weighs 135 pounds, I consider her too thin ( slightly), she says she is not, that for her frame she is exactly right. I think she could stand to put on 10 pounds and she would look better. But then again, maybe I'm wrong. Maybe for what we 'do' we're bang on. Etheline is a yoga buff and plays tennis a great deal. I run, kickbox and do weights, I'm also broader and taller than Etheline.
Anyway, the reason I'm asking is because according to Etheline, at least 80% of the women in her office are what she would consider overweight. And by overweight she means carrying anything over 20 pounds for their frame. Because I'm feeling pretty mellow I suggested that we are not autobots and that everyone's body shape is different. Etheline countered with fat is fat, bones and natural shape not withstanding. I said some people can carry off a bit more weight better than others, she said 'bollocks'.
I asked her what the fuck was eating her this morning and it turns out that she overheard one of her colleagues describing her- Etheline- as 'scrawny' and this was what sparked her ire. I said 'oh.'
(I know for a fact she is not 'scrawny' but then read what I said earlier about the extra 10 pounds.)

I think with age you've got to allow a little leeway, everyone 'solidifies' slightly as they get older, I think ten pound over or ten pounds under is not the end of the world. But 10 pounds over or under what? That perplexed me slightly.
So I'll ask it, what you you consider too fat? And on the flip side, too thin?