I got poured a pint by a newbie behind the bar at a hotel recently and she looked embarrassed as it was about 40% head, but to her credit she went to fetch the shift supervisor before I said anything.

He explained after pouring it better that, even the remaining head (It had ~3/4 inch even after fixing it) might still be met by derision by many customers. "They'd be asking if you would be charging them for just for the half" etc.

There's a bit of leeway but you'll quickly hear about it if you short a pint too much.