How much should I trust the predicted WiFi coverage before installation?

Hi everyone,

I’m still learning WiFi design and have one question.

When using a WiFi design tool, how close is the predicted coverage compared to the real result after installing the APs?

Sometimes the design looks very good on the map, but I’m worried things like concrete walls, glass, furniture, and people may change the signal a lot.

Do you normally add some extra margin during the design stage, or do you follow the prediction as it is?

I would really like to know how experienced designers handle this before doing the actual installation.

Thanks in advance.