New Tax Year, maximising Pension tax relief

I am a member of my company's GPP scheme.

My salary is 39,000 PA, age 37, so I pay some Higher Rate Income Tax (40%)

I want to make the exact contribution to this Personal Pension that attacts Tax Relief at 40%, when it falls to 22%, I'd rather ISA that money than tie it up in a Pension.

Googling shows me that the 2005/2006 PA is 4895

Remembering my FPC1/2/3, I reckon that I pay the following amount of Income Tax

So, the slice of my salary that attracts 40% Tax is £1705.

I do an annual Tax return, so can someone finish off the maths and tell me the exact amount I should put into my Pension every month to maximize tax relief.

Thanks.