When you have multiple field kubbs that are left untoppled by the opponent, you may move up to the kubb closest to the Centerline. A temporary "advantage line" will always run parallel to baselines (and the center line) running through the kubb closest to the centerline. Theoretically, a kubb closer to the king could be further from the centerline than a kubb left standing near a sideline. Therefore you always go with the kubb closest to the centerline.
Or...as menbysk states correctly, the kubb closest to your OPPONENT'S baseline (hence the centerline as well).
US National Rules II.B.2 states:
"If the opposing team failed to topple all of their field kubbs on the previous
turn then an advantage line is created running parallel to the baselines and
through the center of the field kubb closest to the center line. While throwing
batons, players may advance to and throw from behind an advantage line."