WOW not easy if you need a prediction of the “distance it is supposed to cover” if indoor. Open free field is 6 dBm reduction per doubling of the distance.
Same setup but different location will give a very different pattern. https://www.youtube.com/watch?v=6hcK9B4HHY8. Even moving the AP half a meter will change a lot.
What is your expected covered distance? What is “covered” anyway, all down to the slowest ‘b’ interface rate of 1 Mbps is still covered, is that down to -86dBm signal?
Practical now: Antenna gain of RB2011UiAS-2HND-IN is 4 dbi , so the antenna are somewhat directional. Stay in the plane perpendicular to the antenna (keep them parallel with each other. 20MHz wide channel will have double the spectral density compared to 40MHz wide channels. Stick to 20 MHZ! Allowed TX power of RB2011 is region related, but the signal of the client device is probably the real limiting factor. (antenna gain helps receiving the weak client signal).