Distance and Bearing Calculations

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Distance and Bearing Calculations

I wrote a python script to to extract call and locator from ALL.TXT file, to use it to calculate distance and bearing from my own location. This data should help to find my actuall antenna pattern by plotting the received stations, weighted by signal level, over an azimutal map.
The problem is, that the distance and bearing differs too much from the results displayed at the front side of wsjt-x.
(approx 5% longer in distance than in wsjt-x and up to 5-10 degrees in bearing at a distance of 2000 km, as far I remember now.)
My questions are:
What model of the earth is used? Ball or Elipsoid?
What formulars are used to calculate?
In FT8 only 4 digits of the locator are sent. Is the target point the middle of a great field?
It seems my own locator with 6 digits is used as start point for calculation and not the middle of a great field.
Where in the source code are this kind of calculations to find to be able to compare the formulars?

Any hint is wellcome.
Michael oe1msb