The calculator works well with Focus Bikes data for example, as they measure the effective top tube (ETT) and fork length (FL) as I would. As seen in the Focus Izalco geometry table: ETT is measured from top of head tube C-C and FL from axle to bottom of head tube C-C.
Other companies measure ETT differently. Cervelo measure slightly below the head tube, along the center axis of the virtual top tube.
The calculator does not account for the difference in measuring the ETT. It can be calculated, but difference tiny, less than half a millimeter. And there's no difference if the seat tube angle and head tube are parallel.