I agree with that original theory. It works in statements and answers but not in the interrogative and not really in the negative. The following exchanges are all perfectly natural for me:
I'm really worried there's something wrong with my sentence. Can you have a look at it please?
Your sentence is fine. I really can't see anything wrong with it. Stop worrying.
How was your meal? You didn't look like you were enjoying it much.
My meal was fine. I'll go back to that restaurant.
Does my bum look big in this dress?
You look fine. Now can we please leave for the party?
The usage in the following sentences is not natural:
I'm worried my sentence isn't fine. Can you look at it for me?
Was your meal fine? It was difficult to tell from the look on your face.
Do I look fine in this dress?
I really can't put my finger on why the first sentences work and the second don't (other than what I already said about interrogative/negative/statement). WE must, of course, bear in mind that the adjective "fine" has more than one definition (see
HERE).
He wore a fine coat of velvet and silk. ("Fine" here means "of very high quality".)
Your sentence is fine. ("Fine" here means "OK, adequate, satisfactory".)