either
nDuration = spell.Level / 2;
or add new line after the original
nDuration = nDuration / 2; (or nDuration/= 2;)
btw I personally using switch for shortened duration (71_SHORTENED_DURATION_OF_DISABLE_EFFECTS) which sets duration of all disabling type of effects to 3 (or 6 when extended)
And this also affects crushing, grasping and forceful hands.
So I suggest to try this switch - 3 rounds might sound too less but seriously keeping player unable to action for longer doesn't add any difficulty only makes it annoying. I invented this switch especially for epic worlds since disabling effects from level 40 monsters usually last 2-4 minutes. In lower settings this switch might actually improve duration sometimes. Check the readme for more info - the switch has three options actually.