I've just always used the amount Paypal converts to at the time of conversion.
I used to do that, then PP changed everything on my account and I couldn't work it out any more.
Still, years ago the first time my husband had to fill in for self earnings, I did the form for him, and the Tax Office didn't accept it, and eventually we got an accountant who said everything I had done was correct, but also found a good amount of money we were each owed from our PAYE employment (I only had PAYE then), so I reckon they earn their keep.
Still, if you want to do it yourself, phone them for correct advice; but like I said, be prepared to wait a while.