> Let some % of candidates who get this wrong through to the next stage and see what happens.

This isn't a good methodology. To do your validation correctly, you'd want to hire some percentage of candidates who get it wrong and see what happens.

Your way, you're validating whether the test is informative as to passing rate in the next stage of your hiring process, not whether it's informative as to performance on the job.

(Related: the 'stage' model of hiring is a bad idea.)