Head Smash is base accuracy 80%. The fog modifies the accuracy of the move by a factor of 60%, meaning the accuracy is 80% × 60% = 48%.
Minimise being used twice means an evasiveness of +2 (it only raised the evasion by 1 stage in gen 4), which is effectively multiplying the accuracy by 60% yet again. The formula is:
base accuracy × (accuracy - evasiveness)
with any extra effects (e.g. fog) applied to the base accuracy.
The 48% accuracy in addition to ×60% from minimise results in a 28.8% chance to hit. Not so lucky after all, and sumwun was correct.