The metric of the cohorts analysis shows, how the conversion rate developed per month since the first visit.
Use the metric with the dimension "Cohorts (Monthly)".
Month 0 = The conversion rate within the first 29 days.
Month 1 = The conversion rate within the 30.-59. days since the first visit.
Calculation: "Qty Orders (Lifespan Month N)" / "Visitors (Lifespan Month N)" * 100
Extensive information on cohorts in Webtrekk can be found in the corresponding training chapter.