CGT on US Shares

I have held US shares in one company for 10 years and for each dividend payout I took the option to take quarterly dividend shares so have acquired shares on 40 different occasion . I have used HMRC's monthly exchange rates to convert the $ purchase costs into pounds for each acquisition. I sold these shares on 19 May 2012 and have CGT to pay. For the sale exchange conversion I have again used HMRC's May 2012 monthly exchange rate for both the sale proceeds and the sellers commission, however the figure I end up with doesn't match with what ended up in my bank account, in my favour. I'm guessing this is a swings and roundabouts thing and by using a monthly calculator there will be variations in exchange rates that cancel each other out. Am I correct to use HMRC's monthly exchange rates for both buy and sell transactions?

Hi Anghammarad Best,
You can use that or any of the 3 spot rates that are available for the tax year in which the gain arises - UK Integrated Online Tariff: 
Trade Tariff: look up commodity codes, duty and VAT rates
Thank you