Imo, time spent on estimating would usually be better spent on design review and careful thinking about what the product should actually do. In a project with clear goals and vision, it's usually not hard to see what needs to be done.
Usually the hard choices are what you have to cut. It'll be clear what needs to be done, you just won't have the resources to get there.
Then there are the "researchy" type projects, where you have a demo system and maybe some expert help from corporate research, and your job is to take some flighty technology held together with an unholy mashup of F#, MatLab and a depth camera and turn it into a consumer product, something people will want to buy for a hundred bucks. The first half of the project is figuring out if it's possible, the second half of the project is getting it working, the third half is polishing and shipping.
("Every project can be divided into three equal efforts, each consisting of ninety percent of the work.")
I look back and see that there were some managers I've had who were masterful, but shamefully I can only appreciate their talents from here in the deep future.
