A properly made flat white is built on a double ristretto — 18 g (0.6 oz) in, around 27 g (1 oz) out — rather than a normal double.
Why it matters
A flat white is roughly 1:3 coffee to milk. A normale at 36 g (1.3 oz) in 130 ml (4.4 oz) of milk is noticeably diluted; a ristretto at 27 g (1 oz) is more concentrated and holds its own.
That is why a flat white made outside Australia and New Zealand often tastes of warm milk: the base was too long.
The dialing implication
A ristretto usually wants a slightly finer grind than a normale, so the shorter yield still takes the right time. Simply stopping a normale early gives you two-thirds of a shot, not a ristretto.
The milk
Stretch for three seconds at most, then spin thoroughly. You want a thin glossy microfoam folded through, not a foam layer on top.
The other consequence
Because the coffee is concentrated and the volume is small, the espresso has nowhere to hide. A flat white exposes a bad shot in a way a latte does not.