Track vocabulary learning per week.
Vocabulary does not compound — you cannot learn a percentage of the words you already know. It accumulates linearly at the rate you study, discounted by what you fail to retain. Forty words a week at seventy percent retention is twenty-eight net, which over six months is about 730 words rather than the 1,040 the study log would suggest. Showing the words lost alongside the words kept is the point: raising retention through spaced review is usually cheaper than raising the study rate.
Retention
Words retained = new words per week x weeks x retention rate
Vocabulary
Final vocabulary = words already known + words retained
Around 50 to 60 percent for passive list learning and 80 to 90 percent with spaced repetition and use in context. The gap between those two is larger than any plausible increase in study volume.
Roughly 2,000 to 3,000 word families cover most everyday conversation, and 8,000 to 9,000 approach comfortable reading of unedited text. Progress is fast at first because common words are the most frequent.