With the last version of JfControls the calculator should be executed as follows:

procedure Calculator;
var
    ANumber: Extended;
begin
    ANumber := 1000;
    JfCApp.Calculator(2,ANumber);
end;

With older versions, you will need to do the following:

procedure Calculator;
var
    ANumber: Double;
begin
    ANumber := 1000;
    JfCApp.Calculator(2,ANumber);
end;

where 2 is the number of decimals.