So just trackback the cordon/cane from the farthest end and cut to a downward bud where the growth is less than a size of a pencil?
Do grape laterals have basal buds like peach/nectarine where I can cut the potential spur an inch or less if the gap between the cordon and the lateral (future spur) is longer than desired.
Edit: It looks like Richard has spur pruned closer to the cordon and has got regrowth.