I've been trying to find a definiton and found a pretty good one from the cervelo site
"frame stack is the vertical length from the BB to the top of the headtube"

Now the trouble I'm having is trying to figure out how stack is measured, or rather in which angle.
My assumption is that stack is measured with the fork installed and with theorethical wheels attached. Thusly the actual fork length would affect the stack measurement and so a longer fork would increase the stack number.
But what confuses me is that some stack measurement illustrations either have the frame skewed in a weird angle or, actually do not incorporate a fork.
So how is stack actually measured and what variables should be taken to consideration?

I'm asking because my road bike is actually a converted cyclocross. This makes comparing different frames a bit difficult since I can't (yet) be sure how fork length in accordance with stack and head tube length affect my comparisons.