Bicycle Mechanics - How do you measure stem length?

Apologies for such a basic question, but I've never thought about it before and after 35 years of bike tinkering I suddenly realised I don't know the answer - or maybe I did know years ago but forgot it! Can anyone help?

If I buy say a 12cm (120mm) long stem, is the length measured:

a) from the centre of the steerer to the centre of the handlebars?

b) from the centre of the steerer to the front of the handlebar clamp?

c) from the back of the stem to the front of the handlebar clamp?

d) from the back of the stem to the centre of the handlebars?

e) Some other way?

It's never been important before, but now I'm having to sort out my correct position on the bike again after a back problem and have 'mocked up' the bar position I need. I now know the measurement I need from the centre of the fork steerer to the centre of the bars with an 8 degree drop (without a stem in place) and need to buy the right bit of kit to fill the gap.

Advice will be gratefully received!

Stems are measured like a. From the center of the bolt to the center of the handlebars.

but this is not always the case. From Sheldon (pbuh)

The "reach" is the distance from the centerline of the steering axis to the center of the handlebar clamp area, but the issue is the angle this measurement is taken from. Traditional "road" stems shaped like the number "7" have a reach measured along the horizontal axis. Newer type stems that have different "rise" angles most often measure the reach along the axis of the rise, which leads to confusion...a 120 mm "7" shaped stem raised to place the handleabars at the same height as a nominal 120 mm stem with 135 degree (45 degree) rise will actually place the handlebars substantially farther forward.

That's brilliant, thanks!