Ask HN: How I can practice open ended “estimation” questions

I've been doing a couple interview post mortems with myself and the common stumbling block across businesses are the open ended questions. "How much money did GMail make last year?", or "How would you run a candy shop?".

Does anyone have advice or literature that could help me with. 1. Ordering my thoughts into a logical framework. 2. Explaining that framework to the interviewer. 3. Not panicing under the pressure . I know about MECE (Mutually Exclusive, Cumulatively Exhaustive) but i don't know if that would work in the short timespan of an interview. I would definitely appreciate any advice!