0.500% of 0.7988 is
0.005 • 0.7988 = 0.003994
The absolute difference between Kelvin's measurement has to be within this threshold. The absolute difference between what he found and what's worth an A is
|0.7925 - 0.7988| = 0.0063
but this is larger than 0.003994. So Kelvin did not make an "A".