Using developer when dying hair

Hey there, i'm looking into highlifting my highlights and just wondered if when using the developer, I have to use the whole bottle? It might sound a silly question, but when directions say use ratio 1:2, does that literally mean just 2 times as more developer than the colour and not just 2 bottles of developer against one bottle of colour?
If so, how do i go about measuring the 1:2 ratio?
Thanks in advance!