@BrianWealthall you don't arrange based on distance but based on SNR (Signal to Noise Ratio) use any wifi tool to measure SNR and allocate any point at any location with at least 24db  (+30db runs perfect) of SNR. In this way you got best performance with Deco's. 

I have 2 story home with 4500sqft with 5 Decos and runs smoothly anywhere in the house

@leogz Many thanks. I'll give this a go. 

This is a decent answer, but the signal strength can be different between different kinds of devices at similar locations. It’s better to have the Mesh device measure what it perceives the S/N to be between the other mesh device. I see in the Deco app that when I click a satellite node, it shows the strength of the connection (for me) back to the base that is wired to my ISP. That picture shows 2 of 3 blue on the signal strength. I’m getting real world internet speeds from OOKLA 65 to 70 Mbps over my 300/300 Mbps ISP from this node. When connected to the base node wirelessly with the same device I can get above 200 Mbps. So is the Deco trying to tell me that the satellite M5 should be moved closer to the base M5, if I want to get better Wifi speeds when connected to the satellite node?

Granted that you don't arrange Deco units based on distance but based on SNR, however for my application I wanted to install/extend my mesh wi-fi from my house to a rear self contained granny flat approximately 10-12 metres away from my house. 

I am proposing a Deco X20 (3 unit) set-up - One Deco X20 unit situated as a base unit connected to my modem, another Deco X20 unit close to the rear of my house and a third Deco X20 unit would be situated within the granny flat.

There are large glass windows/doors in both my house and the granny flat and the maximum distance between the 2nd and 3rd units would be 10-12 metres with the only hindrance being the two glass boundaries between the the 2nd and 3rd units.

Prior to investing in the necessary components and perhaps being disappointed, would someone being able to advise if this proposed set-up should allow me to extend my wi-fi over this distance to the rear flat or can you offer an alternative arrangement (I'm not keen on a wi-fi extender).

Thanks & regards,

@Plebo2000 

Nothing beats the actual test of signal strength. If you have good WiFi signal at the future location on 3rd Unit, coming from the 2nd Unit, the changes are good Deco mesh will be working just fine.

To run this test, you won't even need Deco node. The minimum you'll need is 2.4GHz/5GHz WiFi router to put where 2nd Deco Unit will be, and a smartphone to measure WiFi signal from that router at the location where 3rd Deco Unit will be.

I would suggest installing WiFi analyzer on smartphone to measure both 2.4GHz and 5GHz signal strength, because that'll tell you how well Deco can utilize both WiFi bands for backhaul.