Abstract: The method and a device determine a parameter that characterizes a number of spare positions to be used for cancelling the crosstalk of lines joining a digital subscriber line vectoring group. The vectoring includes cancelling the crosstalk caused by a maximum number of lines of the vectoring group and reserving the number of spare positions. In order to provide a method and a device for determining a number of spare positions such that the vectoring group operates efficiently, the method includes determining a residual crosstalk metric that depends on a residual crosstalk between at least two subscriber lines of the vectoring group; and determining the parameter that characterizes the number of spare positions depending on the at least one residual crosstalk metric by successively modifying the parameter until the residual crosstalk metric fulfills a termination condition.