How much battery does FORE use? And how can I reduce battery consumption?

Unfortunately, it is not generally possible to say how much energy FORE is using on your phone.

This depends on various factors, such as whether the phone often has to establish a network connection during the round or which other apps or Bluetooth connections are running in parallel.

But you can use FORE in flight mode if you have started the round or downloaded the course in advance. 

Then the battery consumption is 5-10% per 9 holes.