Two calendars are deliberately mixed here. "Date required on site" is the CPM engine's early-start date, computed on the project's working-day calendar. Lead time is subtracted in real calendar days, not workdays — suppliers don't work your site calendar. That's intentional, not a bug, but it means the order-by date won't match a naive hand calculation using only one calendar. Total float is likewise computed on the project's working-day calendar by the same engine FloatCheck uses — float calculated by the FloatCheck CPM engine.