I would argue that what makes sense when considering temperature percentages wrt dissipation, is the difference between old and new, divided by the difference between the system at rest and the old temperature.
Which is then a ratio of offsets, rather than a ratio of one offset and a difference with an arbitrarily defined origin.
In this case, it is fair to assume the temperature at rest of the system around 292K, or 19C.
Which would give: (78.5C - 70C) / (78.5C - 19C) = 14.29%, or (351.65K - 343.15K) / (351.65K - 292.15K) = 14.29%.