Preferential exchange rates for foreign dividends

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!!

Hi Stevo3475
You will need to use the same date for both conversions
You can find rates of exchange at: BIM39505 - Foreign exchange: rates of exchange - HMRC internal manual
Thank you