This antenna has 6dbi according to specs.
http://routerboard.com/RBMetal2SHPn So you should use regulatory domain, your country and put 6 into the antenna gain field (in winbox see wlan interface, wireless tab, advanced mode). After that the radio will be transmitting with the correct power. Do it with all of your access points and then start your measurements.
Now to your question. Wifi is not like radio broadcasting system that depends on power of transmitter and selectivity of receiver. Wifi is bidirectional, so it has transmitters and receivers on both sides. If you want to connect the mobiles, take into account their transmitting power compared to the power of metal with such huge antenna. What it means? While you will get full signal on the mobile, you will not be able to connect to the accesspoint just because the accesspoint will not hear the mobile or will not be able to distinguish its signal from the noise.
There are some possibilities how to solve this problem:
1) use panel/sector antennas instead omnidirectional (in case of big halls or open spaces) smartly placed around and not aimig directly against each other.
2) use more widespread accesspoints with low gain antennas and very low transmitting power forcing the mobiles to roam instead of holding it connected to accesspoint just because the mobile gets still strong signal from ap regardless the ap barely hears the mobile.
Have you seen this page?
https://en.wikipedia.org/wiki/IEEE_802.11#Protocol Everyone should before wanting to get 300m indoor link between ap and mobile phone.
(on the opposite, I managed to have almost 1km link between groove with 7dbi omnidirectional antenna and a mobile phone)