751G Antennas

Hi Guys

I’m a complete newbie to Mikrotik, but bought my first 751G the other day. I need to set the card up on a farm in Zimbabwe, to get the highest possible coverage (wireless transmission power laws are never going to be policed out here) . I intend to use the MMCX adapter to connect an external omni antenna. But I need some advice. I have the option of an antenna with reverse polarity? I have heard that most wireless routers use these - is there an advantage? Once this is decided, where do I input the antenna gain into winbox? Do I add a negative sign in front of it? OR if i leave the gain at the default of 0 will my theoretical power output be higher?

Thanks in advance for any assistance!

How long distance do you want to cover with 751G?
751G has good enough internal antenna (for purposes it was built).

Easily distances over 50 meters. What is the gain of the built in antenna?

2.5dBi, but it has 1W wireless transmit power. 50m should be OK