So lets play Ballpark Mathematics.
Assumptions:
- 14,000 active detectorists
- 2,400 responsible detectorists
- 30,000 * 0.68 = 20,400 finds reported by detectorists last year
And so
(2400 / 14000) * 100 = 17.1% of detectorists are responsible
And (14000 / 2400) * 20400 = 119,000 finds found last year that weren't reported by detectorists.