John Leathem admitted killing the 15-year-old when she stopped for a breakfast roll at his Delicious Deli in Clydebank, West Dunbartonshire, on March 19 last year.
He was given a mandatory life sentence at the High Court in Glasgow in October and ordered to spend at least 27 years behind bars, but launched an appeal against the minimum jail term later that month.
Senior judges at the Appeal Court in Edinburgh have now quashed the original punishment and imposed a 23-year sentence.
The opinion of the court, delivered by Lord Turnbull, reiterated that Leathem had committed a “most appalling murder” but said other circumstances needed to be given “proper weight”.
At a hearing in December, Leathem’s lawyer Iain Duguid QC said the punishment part of his client’s sentence was higher than other child murder cases and “represents a significant increase in the tariff” for such offences.
He compared the sentence handed down in other cases such as the 23 years given to Alexander Pacteau for the murder of Irish student Karen Buckley.
Ruling on Leathem’s appeal, the judges said: “The appellant was a family man of previous good character who had not offended before and who had expressed remorse. The murder was not premeditated.
“When these circumstances are all taken account of, we think it can be said that the length of the punishment part selected in this case was inconsistent with current sentencing practice and was excessive.”
Leathem, 32, stabbed the defenceless teenager 61 times and inflicted a total of more than 140 injuries in the attack.
Despite claims he was in “a panic”, he cleaned up his crime and dumped the 15-year-old’s body in bushes near Great Western Road.
Paige’s mother, Pamela Munro, last year described the appeal as “cruel”.