Pension (or an annuity) in payment

The post share pension in payment is simply reduced in proportion to the % shared

Example

Pension in payment pre share = £10,000 p.a.
Percentage share = 40%
Pension debit = 40% x £10,000 = £4,000 p.a.
Pension post share = £10,000 less £4,000 = £6,000 p.a.