1/3 works well for tasting flights. If you go to a CAMRA (Campaign for Real Ale) beer festival they should sell you a pint glass marked with 1/3 and 1/2 lines too (and a line at 1 pint with an extra cm or so for head). So you can choose whether to taste many beers or have full pints of a few.
The UK also allows bottled beer in whatever is marked on the bottle too - it’s only the decanted stuff that needs to be measured.
I remember being in a bar in the USA with friends where I needed to drive but wanted to try a local beer, so asked for a half pint… They couldn’t do it. So I let one of my 20 year old friends finish the pint.
