Apropos heads on beer, because I have arthritis in my thumbs, I thumbtimes find it hard to get a grip on a straight pint glass, and I'm not getting into drinking two-handed yet! So, I asked whether I might be allowed to bring in my own tankard so I can hang on to the handle without falling off it. It is oversize but certified 'Pint to the Line'. The staff nearly always fill her to the line, including the head. But when it settles, it can be about 10-12% below the line.
Once I got a full pint, but that was when one of the bar staff left the tap on a bit too long (I drink Extra Stout). I have never complained, nor would I. For the price of Extra Stout compared with Guinness elsewhere, I don't mind my glass being underfilled, and I always have the option of asking for it to be topped up anyway. I don't bother.
But, this probably illustrates that when staff are trying to achieve the 5%, many customers are probably only getting about 90% of a pint. If they were trying to pour a full pint with a head, then customers might get the 95% of what they've paid for.
Guinness poured correctly in Ireland is done in two stages. The glass is half filled, with the beer running down the inside surface of the glass. It is then left to settle for ten minutes or so, before being carefully topped up. Due to the high turnover, waiting time is reduced by lining up half-filled glasses.
I can't see HS allowing that! But, as an ex-pat suvverner in York, I reckon its all down to this norfern habit of drinking bubbles. Now a good warm flat pint would fill the glass every time!