I think the following formula is used:
(Pay for fortnight less [2 x weekly contributory OAP x factor**]) x 3.5%
Plus
(1.5% x pay for fortnight)
** The deduction for the contributory OAP amount must "match" the frequency of his pay- so, for example, if he's paid fortnightly, you take twice the weekly contributory OAP and multiply that by 2 again. If he's paid monthly, you take twice the weekly OAP contributory amount and multiply that by 4 (not sure of the exact figure, probably a little more than 4) and so on. In your husband's case, as he's paid fortnightly the factor value will be 2.
I'm not sure of the current weekly OAP amount- I think it's €230.53 but it may have changed.
The result should match his payslip.