Once you last bar starts flashing you have about 7 litres of fuel left, which is just less than a third of you tank. If you reset your trip when you filled up you have about half that distance available to you (eg if you have done 160 miles you can do a bit under 80 in reserve). To add a bit of safty margin I'd take 10 miles off that, and another 10 if you brim the tank when you fill it, ie 60 miles in reserve in this example.

In practice I could always guarantee getting 200 miles from a tank (even when getting only 40mpg due to stop/start and short runs). My lest efficient tankfull (39.88mpg) gives me a range of 202 miles to a non-brimmed tank.

The closest I got to running out was after 192 miles (about 40-50 on reserve) I only got 21l in the tank (not brimmed) which means I had about 2l (or nearly 20 miles) left.