Interpolate, don't ever extrapolate. Extrapolation will normally void the insurance, leaving you in the wind to pay for any and all damages should you have an incident or accident.
Let's be clear too there is never, ever, a reason that you HAVE to go flying. If you're flying for pleasure, then have enough money to park it and go commercially. If you're flying commercially, have the fortitude to say "no" if you cannot back up your flight with actual, out of the book data.