I have some very old muscadine(?) grapevines that are growing on two wires, one above the other approximately 2-3 feet apart, running about 100 yards long, somewhere around 10-15 feet between each actual plant base. Last winter they were cut back to the main stem that runs on each wire. Now they are "bushing" out considerably and need pruning. Can someone give me some basics on how far, etc to trim them back?