@Salvstar 

Hello, sorry to tell you that there is no such standard available cause the distance varies due to the different network environment and requests. Back to your case, to keep the stabilization of the connection between each Deco unit, it is suggested to locate the slave Deco where you can get good signal strength from the front-end device, like 2 bars.  

And considering the Deco P9 supports PLC backhaul, you can adjust the wall sockets to find the best location. 

May it help.