Measure daily productivity.
Productivity measured only by tasks completed rewards padding the list with trivial items, so this blends two things equally: what fraction of the plan you finished, and what fraction of your day was genuinely uninterrupted. Interruptions then apply a penalty at roughly five minutes of recovered attention each, which is why a day with twelve interruptions loses an hour before any of them is counted as work. Completed tasks are capped at planned so the completion rate stays interpretable.
Components
Completion rate = tasks completed / tasks planned x 100; focus share = focus hours / hours at work x 100
Score
Score = completion x 0.5 + focus share x 0.5 - interruption penalty x 0.3, bounded 0 to 100
Because completion alone is gameable and says nothing about the difficulty of what was done. A day of ten trivial tasks with no focused time rarely moves anything important forward.
Three to four hours of genuine deep work in an eight-hour day is strong for most knowledge roles. Consistently claiming six or more usually means the definition of uninterrupted has slipped.