Back when I were a barman, the Tetleys hand-pump dispensed a quarter of a pint per pull. Not measured, but it did. So roughly 4 pulls per pint, but you judged it rather than counting. Back then, you over-flowed the glass until you saw the cloudy yellow bubbly liquid rather than white froth. This then settled out into a lovely 5-10mm of smooth head on the top, nobody ever asked for a top-up.
But nowadays they only carry on filling until the froth reaches the top, which is a con. But there are reasons for this. For one, the overheads of pubs are huge, so they're forced to steal from customers or go bust.
But also back in my day, every hour or so the landlord walked along the bar with a big stainless bucket. He tipped the drip-trays of all bitters and lagers into this - everything other than guinness and cider. Then this bucket was poured into the Tetleys barrel. So you could overflow a glass as much as you liked, it didn't cost anything as nothing got chucked away. This was back in the day when customers would give you back their spit-filled glass they'd been handling after going for a pee and not washing their hands and you'd refill it. Probably after going for a pee and smoking a ciggy. It was disgusting really, but to be honest that pub had the loveliest pint of Tetleys I've ever known. All the drip trays went in the glass-washer at the end of the night, so they were clean each day.
But I'm sure that what happened then is definitely illegal now. I haven't even seen a pub refilling a glass for years now, probably all banned. So I'm sure that recycling the drip trays will be illegal too. It possibly was back then anyway, but it happened.
So every drip of froth that overflows the glass is a loss to that pub now.