Stevo3475

Hi,
I see various responses in tax guidance saying that it is permissible to use a preferred rate of exchange when converting foreign dividends (out of annual/monthly/spot/daily rates).
For my foreign dividends, I have values for the dividend amount and then the withheld foreign tax, on which I can claim relief. Is it permissible to use preferential exchange rates when converting both the dividend amount AND the withheld tax for entry into my self-assessment form (those two exchange rates will of course be different)? OR, must I take the converted dividend amount and then calculate the foreign tax withheld using the foreign tax rate on the dividend amount (15% in this case)?
These two methods will produce slightly different relief values and the former method is preferential to me but I need to know if this is permissible. The self assessment form will automatically do the latter method but gives an option to calculate manually.
Thankyou!!