Measure, mirror, inspect and guide. I remember trying this out years ago and wondering whether I'd find a use for it. I shouldn't have worried, within a couple of weeks I couldn't imagine my Mac without it and it gets better with every version.
I think this is actually my most used Mac app ever, I don't even remember life before Soulver. Everything from UI calculations to planning how much money we might make, this is the tool I reach for.