Just checked my (sad anorak) spreadsheet and in the last 52,000 miles the maximum amount I've put in at one time is 51.27 litres, but I don't think that I've ever gone below about 35-40 miles remaining. Having said that, there's already a known discrepancy of about 5% in the trip computer as the average MPG is always out by this amount from the measured MPG on a tank for tank basis. That reduces the 10% to a 5% safety margin which seems more realistic.
Alan