This one could be done without even using a calculator.
With this particular stopwatch, the smallest increment is seconds. Under ideal conditions, it can be read down to the nearest tenth of a second -- more realistically, the absolute uncertainty would be about 0.2 s.
If we use that abs. uncertainty -- st = 0.2 s -- in our calcuations...
st / t = (0.2 s) / (27.3 s) = 0.007326...
...which is closest to 0.005 (or 0.5%).