There is a handy gauge fitted to most modern cars that'll help.

Also, I've done 10+ miles when the DIS says 0 with no problems. It rounds to 5 mile increments, and is based on recent mpg so you can increase it by driving more gingerly. I've started a drive from Edinburgh with 100 miles on the DIS and 150 miles later still have 70 miles on the DIS.