If you were concerned about regulating the amount of milk you were adding, tea first wouldn't even be a possibility.
If you're concerned about regulating the tea (to water) to milk ratio, i.e., obtaining a reliable flavour), milk first wouldn't even be a possibility.
Most people use their teacups more than once.
Even among people who don't, disposable cups, having been mass-produced, are all the same size.
Further the milk first method cannot account for differences in tea strength across brewings.
If we take the amount of times/chances milk first could lead you astray vs. the amount of times tea first plus stirring as you pour could lead you astray, the former is oviously greater and more likely.
I saw somewhere here somebody talking about the different ways the milk sugars might caramelise with the two methods being a factor, and obviously the article suggests there is a difference between the two methods in terms of flavour, but in terms of reliability of flavour, the latter option is obviously more reliable.
I take my tea black and unsweetened anyway, unless I'm drinking Hong Kong style milk-tea that is... then all bets on my blood-sugar levels are off...
Drinking vessels come in all different sizes, from dainty little tea cups to mugs that hold an entire litre.
Measuring the milk requires both measuring the steep time exactly, and using a separate measuring cup. This wastes time and adds unnecessary cleanup.
Stirring as you add milk to the tea allows you to tell exactly the right amount for that cup (and for how long the tea was steeped) based on the colour of the beverage.
> measure a specific amount of milk in isolation than to eyeball how much of it you've just added to something else
Think of it as the difference between measuring a liquid in a container and eyeballing a liquid in another liquid.