I can understand why this happened (and still does) as catalogues do not give enough information on this subject. The majority of specialised catalogues just state that "all short and inset bands must be clear of the perforations", which to my mind sends out a wrong message that any shift that is clear of the perfs is good enough to qualify.
I prefer the Deegam criteria for these listings as it is precise and plain. In his handbook Douglas Myall has tried to educate collectors and dealers that a perforation shift only gives the illusion that stamps have a phosphor shift. Only perfectly centred stamps qualify for listing in his handbook. Due to Douglas's research the editor of the MBPC (Modern British Philatelic Circle) has now adopted this same policy.
Douglas writes " The test is: if the perforations had been where they should have been, would the phosphor still have missed them. If the answer is yes the shift will qualify for listing in his handbook".
In short Douglas also writes "To be clear of the perforations the gap between the edge of a phosphor bar and the centre of the perforation hole MUST be .45 mm (half the width of a hole ) or greater, as a phosphor shift to that extent is permitted anything less will not clear the perforations".
Please note: Not all phosphor shifts are errors. Certain printings exist from se-tenant and barcode booklets where the phosphor alignment was placed away from the perforations purposely, these as single stamps are still known as inset or short bands but the actual booklet panes are classed as interrupted phosphor. Perhaps myself , Larry or even Robert Bostock (hint hint Robert) could make this the topic of a future post.