I use NetSpot no see witch place is better for any Deco spot.
https://www.netspotapp.com/

Sometimes you have great signal and bad noise ... netspot mensure this and you can avoid any trouble.

If I was setting them up again, I'd do this:

1. I would set up Deco #1 and test my speed (using Speedtest.net app on my phone) while standing right next to it.
2. I would go to roughly the next part of the house where I want to put Deco #2 and see if the speed tests roughly the same, while I am still connected to Deco #1. If it does, that's where I would put Deco #2. If not, I'd move myself closer to #1 until the speed tests the same as in step 1.
3. For Deco #3, I would turn off wifi on my phone, and back on again, while standing near Deco #2. Then I'd move to the next part of the house and do the same as in step 2.

This made sense when I did it at my home because the Decos are all in a line, from front to back of house.