I have frequently felt robbed at the pub. You know the feeling. Tapping your card on the reader and seeing £6.70 drain from your bank account in exchange for the most average glass of IPA in the world; doing it again, a third, maybe even a fourth time, shuddering internally. News last week, however, confirmed the worst: actually, you really have been getting robbed at the pub.

Last week, the Chartered Trading Standards Institute published a study of drinks served at 77 pubs and bars around the UK. They found that, out of 137 orders of pints, half pints and 175ml glasses of wine, about 70% contained less drink than they were supposed to; 29% of these short measures were under by 5% or more. Beer was where there were the most discrepancies: 86% of orders were short-measured. For the average beer drinker, these losses add up to £88.40 a year of beer paid for and never actually served.

There is, I can’t deny, something funny about the notion of clandestine agents roving the country’s drinking houses and measuring their pours down to the millilitre. But let’s take these findings seriously. Let us imagine that this is, in fact, one of the most pressing issues facing consumers. And let us focus specifically on beers, as this seems to be where the problems are greatest. What exactly a perfectly poured beer should look like is a tricky question. Some people like a bit more head on their pint of lager than others. Go to any pub in east London right now and you will find three graphic designers willing to talk to you for up to an hour about exactly how much foam there should be on top of a Guinness.

This argument is reflected in the survey: 23% of people felt that the head should be included in the pint measure, and 35% felt it shouldn’t. On the other hand, the Campaign for Real Ale (Camra) – the UN security council of beer-drinking – maintains that the pint measure should not include the head, and that consequently consumers have a right to what it is pleased to call a “100% liquid pint”. These are divisive issues.

Some people will happily just look at their pint, see it’s a little short, and ask the bartender to top it up. I’m not one of those people. I just can’t bring myself to do it, except perhaps in really egregious cases of underpouring. This is because there is a human being standing there who just gave me the pint. We have a culture of OK service in the UK. We don’t generally go in for the thrilling rudeness of, say, Parisian waiters, or the obsequious attention you get from US servers. I like it this way. I don’t want to feel actively shunned by service staff, but I also don’t want to feel like there’s a grim-faced manager in the back office whipping my waiter to smile at me more.

And when I worked behind a bar, if someone asked me to top up their beer, especially if the pub was busy, and especially if they did so with a look in their eye that implied I had intentionally shortchanged them, I hated it. I hated them. Oh, do excuse me, did sir want a thimbleful more beer? Will that be all, my liege? Does master’s pint meet with his approval now?

Now, from the other side of the bar, and in the face of the concrete figure of £88.40 in lost beer, I sort of see the point of those who ask for the top-up. You buy a pint, you should get a pint. A simple and fair exchange of money for a specified good. If people didn’t get a completely perfect, 568.261ml of imperial pint when I was serving, it was because I wasn’t careful enough in pouring it. Fair enough. I realise now that I mostly wasn’t angry at them. I was angry about being on my feet all day, and the smell of the storage room in the basement, and having a manager who made us watch him do magic tricks and took too much interest in my colleague’s recent boob job.

But … while it might be correct to the letter of the law to get a full imperial pint every time you order one, it does not feel true to the spirit of pints to quibble about it. Of those 86% of beers that were underpoured, the average deficit was only 4%. We are talking about less liquid than a single espresso. Sure, if someone hands you a beer that is visibly shit – three inches of foam and a clearance of air in the glass on top of that – by all means ask them to finish the job. And if you’re in one of those cool beer-bore places where a man in an ironic cap smugly serves you a “schooner” of mango sour for eight complete pounds, it is going to rankle not to get a full glass. All right.

But for marginal cases, suck it up. A pint is, to a point, an idea. It is a lovely yellowy brown glass of “having a nice time”. The exact measurement should, rightly, be neither here nor there. Put the tape measure away, and enjoy your beer. Cheers.

• Imogen West-Knights is a journalist and writer. Her novel Deep Down is out now