watchman oil tank sensor information

[kisoro]

I have a Watchman oil tank sensor which was installed a few months ago with a new tank outside (when I wasn't around) but no instructions were left with it. The screen now shows just two bars with a downward flashing arrow indicating that the oil level is low. Can anyone tell me at what point on the screen do I need to order some more oil. With the last tank and sensor, when I stopped the central heating and only used the oil for hot water there was plenty of oil in the tank for months and months. I would prefer to order more oil once the cost of oil drops.