As a point of reference - the cliffs of Dover can be seen from Calais on a good day, and that's a distance of 34 km (21 miles). Wikipedia gives the max. elevati

As a point of reference - the cliffs of Dover can be seen from Calais on a good day, and that's a distance of 34 km (21 miles). Wikipedia gives the max. elevation at Calais as 18m, and 107m at the White Cliffs, so that works out nicely (though the calculations would be a bit more involved than what the calculator you link to allows).