I'm pretty good at budgeting and estimating because I've been at it a long time. The 15-20% are just things that go sideways -- usually on the client's side and

I'm pretty good at budgeting and estimating because I've been at it a long time. The 15-20% are just things that go sideways -- usually on the client's side and I typically make a snap judgment not to get into a scope/billing dispute because being easy to work with is part of my brand. And I find I can often avoid detailed budgeting and scope, controlling the latter via project management once the engagement starts. (I have a pretty sophisticated spreadsheet if I need to get deep in the weeds on estimating, but it's a relic of when I did time-and-materials work.) Example: If past experience says a project is going to take about X thousand dollars per month worth of effort spread across Y number of months before we're likely to see results, then my whole budgeting and project definition processes come down to understanding the business metrics success will be judged on and building scope around that. We can manage task backlog and progress reporting with weekly check-ins.