Two weigh-ins is all it takes. No working out your own rate.
This is a straight line, and bodies aren't. It assumes the pace you've averaged so far simply continues. In practice weight loss slows down. Every published trial curve bends, so the further away your goal, the more optimistic this date is. Treat it as the earliest plausible day, not a promise, and don't book anything around it.
Tirzing fits a trend line to your last eight weeks of weigh-ins rather than two points, so the date moves as your pace actually changes, and it comes with an earliest/latest band instead of one hopeful number.