£50 a time. Nowadays, I normally only need to fill up each car once a month, twice at the most. This takes me from around 1/4 full to almost full, which avoids the problems associated with a low fuel level whilst maintaining an adequate reserve.
In the past, I often needed to fill up several times a week, and then I would brim the tank to the nearest £. I appreciate that 'brim to brim' also yields an accurate consumption check, but entering my purchases onto a spreadsheet enables me to calculate my MPG even more accurately.
I find that the round number of £££ makes life easier when it comes to reconciling credit card statements and preparing accounts.
Regards, John.