The rule of thumb we use on this side of the pond is if its less than 10% contaminated then you will be OK - any more than that and the advice is to drain the tank. I cannot vouch for the scientific substance but the UK motoring rescue organisations follow it.
To my eternal embarrassment I did this about a year ago and before I realised I had selected the wrong fueling line I had put about 30 litres of diesel into the almost empty tank. Phoned the RAC (rescue outfit) - they repeated the above mantra and offered to tow me free of charge to a garage about 15 miles away - where I would be charged around £60 ($90 ish) to have the tank drained and the 'contaminated' fuel disposed of in accordance with the environmental regulations' - yeah right! As it happened I was only 3 miles from home - lift me home please - and I drained it myself. Unfortunately I have no kit that runs on diesel and I had to dump it at a recycling centre (no cost though).
Trips to the filling station have never been the same since and I am now paranoia personified - talk about measure twice - cut once