It’s essential to know how much to pour into each glass.
It would be easy to keep a mental reference point on a glass if all glasses were of the same shape and size. But alas, nowadays glasses come different shapes and sizes.
A standard pour is about 10-12 centiliters. That itself is hard to gauge. So go for the fill level that corresponds to the widest circumference of the glass. Many stemware manufacturers have designed glasses with as the gauge for the correct fill level.
At this level, the wine’s surface area that’s exposed to air is at it’s maximum. Aerating the wine helps released its aromas and ‘soften’ the tannins in the wine. This level will also leave plenty of space for you to swirl your wine. Swirling further aerates the wine.
If it’s hard to find the level that has the widest circumference, then just fill a third of the glass. This level should allow the wine to aerate well and still leave plenty of room for swirling.