In my experience, whether in UK or Hungary, the estimated coverage on the tin is always an overestimate, i.e. it never covers as much as they say it does. Of course it depends on the porosity of the surface etc but I have never got close, even with good-quality paint. I tend to use a trade white emulsion first which is quite cheap just to get the coverage, then put on any colour or specialist paint afterwards, because that works out cheaper than using two coats of the "one coat" paint etc.
I wouldn't have bought a 5 litre tin... instead I would have bought two 2.5 litre tins. Then, if I were lucky, I would have a 2.5 litre tin that I could return or keep for another job. Yes, it's slightly more expensive per litre that way, but saves having to throw away a lot of paint - after the can is opened, the air in it will eventually harden the remaining paint unless you can expel it e.g. by inflating a balloon in the can or finding another suitably-sized container to decant it into.