cap gain on foreign shares

I intend to sell some US shares of mine at a reasonable gain. I intend to sell such a number that will just keep me below the capital gain threshold. I think this is 8200 UKP for 2005/6, Can someone please confirm that figure.

Is the capital gain calculated as:

1. sale price in $, minus purchase price in $, converted to UKP at time of sale.

1. sale price converted to UKP at time of sale, minus purchase price converted to UKP at time of purchase.

2. My choice.

1. Something else.

TIA.

Fred