Dear Fjns, I've noted that apparently you gave me the same function as suggested previously, but this time the 20k lines calculation went surprisingly and satisfactorily fast:
2 seconds by changing the minimum selection limit value, 4 minutes to eliminate or insert a line, also 4 minutes to change a formula in a cell! Which factor has triggered such a beautiful result will remain for me a distressing mistery! Yet the solution of this very penalizing problem of mine make me greatly happy. ... also the solution suggested by P45cal worked equally fast ... therefore a very very very big thank to both. Your grateful AP