Enter what you know about your own numbers. The calculator shows the leads, revenue and return an SEO investment would need to produce to be worth it, and the month you would break even.
Assumes traffic ramps rather than arriving all at once. Cumulative revenue against cumulative spend.
The maths is simple: traffic multiplied by conversion rate gives leads, leads multiplied by close rate and deal value gives revenue, and revenue measured against spend gives return. The value is in forcing the assumptions into the open.
Two things it cannot model. First, compounding: rankings won in month four keep earning in month twenty-four, so a twelve-month window understates the real return. Second, competition: the traffic figure you enter is only reachable if the keywords are winnable at your current authority, which is what an audit establishes.
Use it to sanity-check a budget, not to forecast. If the numbers only work at a conversion rate you have never achieved, the problem is the plan and not the calculator.
On a free call I will look at your actual traffic and keyword data and tell you which of these assumptions holds.