The Ground Time Overshoot (GTO) is the share of flights exceeding the scheduled ground time during a defined period.
$$ \mathrm{GTO} = \frac{N_e}{N} \times 100 \ [\%] $$
where
$N_e = $ Number of flights with an actual ground time exceeding the scheduled ground time (over period $P$.)
$N = $ Number of flights (over period $P$.)