I tend to agree that more than six strokes is almost certainly excessive, even back in the 1960s and 1970s (the era I am familiar with).
I know nothing about Derek Slade’s canings other than the photocopy of the punishment book page which was published in at least one UK newspaper. However, his use of the slipper (or more accurately training shoe) was deemed excessive by the judge at his trial so one shudders to think what he might have done with a cane, clothed or unclothed (we don’t know but I suspect it was clothed).