It's not calculated anywhere, the user is supposed to use something as a value. Here's a little example of how it's done in one of GLScene's demos:
[pascal]
procedure TForm1.FormMouseWheel(Sender: TObject; Shift: TShiftState;
WheelDelta: Integer; MousePos: TPoint; var Handled: Boolean);
var
f : Single;
begin
if (WheelDelta>0) or (GLCameraControler.Position.VectorLength>0.90) then begin
f:=Power(1.05, WheelDelta*(1/120));
GLCameraControler.AdjustDistanceToTarget(f);
end;
Handled:=True;
end;
[/pascal]