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know how much roadmap-project work people can do in a week. commonly it's 40-60% of their time. call that PR (for Project Ratio).
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make a spreadsheet: [Story, Description, People, Weeks, Person-Weeks].
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break the work down into comprehensible pieces, where you feel confident saying things like "this will take 2 people, working full-time, 3 weeks to implement." in particular, break down milestones that deliver value; where, if the business told you to do something else, that would be okay, and the work done wasn't wasted.
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fill in your spreadsheet columns appropriately (say 2 people, 3 weeks, 6 Person-Weeks).
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sum up the Person-Weeks. ("People" and "Weeks" are for later.)
