The antennas that come with the RB2011 are (in theory) omnidirectional ones, their emission shape is loosely a doughnut, which is fatter for lower antenna gains and slimmer for higher gain antennas, see this picture:
https://www.radiolabs.com/images/omni-antenna-radiation-pattern.png
And this thread for some context:
http://forum.mikrotik.com/t/signal-fall-with-antenna-gain-setting-on-rb2011/137211/1
The normal antennas coming with that should be 4-5 dBi gain.
Being omnidirectional it likely gets more interference/noise in crowded areas from different sources (including - besides access points - cordless phones, microwave ovens, bluetooth devices, and even USB 3 devices, as they are all emitting around 2.4 GHz):
https://en.wikipedia.org/wiki/2.4_GHz_radio_use
I know nothing about the kind of calculations involved, but I have seen several projects on github about this, among them several are Python based, you should probably check what others have come out with, I will cite Mikhail Kalashnikov (the designer of the AK-47) 
:
“A lot of Russian Army soldiers ask me how one can become a constructor, and how new weaponry is designed. These are very difficult questions. Each designer seems to have his own paths, his own successes and failures. But one thing is clear: before attempting to create something new, it is vital to have a good appreciation of everything that already exists in this field. I myself have had many experiences confirming this to be so.”