From HMRC website:
If you contract out and pay into an occupational pension scheme
If you contract out because you're a member of your employer's occupational pension scheme:
* you and your employer will pay reduced National Insurance contributions
* your employer will pay at least the amount saved in contributions into the scheme
* HM Revenue & Customs (HMRC) will pay an additional rebate into the scheme based on your age and your earnings if your scheme is a 'contracted-out money purchase scheme'
A contracted-out money purchase scheme is one where your pension depends on how much has been paid in and on the value of your fund when you retire.
You'll pay a reduced National Insurance contributions rate on your earnings up to the 'upper accrual point'. This is 10.6 per cent instead of the standard rate of 12 per cent (2012-13 tax year rates).
So if you've been contracted out for all 35 qualifying years, you will have contributed in full for the equivalent of (35 x 10.6 / 12) = 30.92 years, giving you a single tier pension of (144 x 30.92 / 35) = £127.20.
Presumably, if you are instead contracted out for (35 x 144 / 127.20) = 39.62 years, you will arrive back at the single-tier maximum of £144.