The generally agreed upon spec is 15.8 for most, if not all, 240s.
It seems like most people's fuel gauges vary somewhat, so you'll just have to go with experience to tell you when to fill up.
You can start to worry when your car starts to stumble upon hard acceleration. When that happens to me, I usually fill up with about 14-14.6 gallons. I let the pump shut off automatically, and I don't top it off. At that point for me, the needle is just starting to exit the orange zone on the gauge, and on the bottom white line.
Beware though, of totally running out of gas, because the in tank fuel pump is cooled by the gas, and it can burn itself out if run with a totally empty tank.
If you've never had your car stumble on hard acceleration, you're not getting close to the bottom yet.
Greg
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