How strong of a signal to reach a repeater from inside a house

[LeoG]

Here's another question I don't actually expect an answer to but figured I'd ask anyway.

I have a repeater about 17 miles away from my house and it's 10 miles from my sons house.  I was surprised that both of us can be inside our homes and talk somewhat reliably with 4.5 watt HTs.  My house has aluminum siding, his doesn't.

Anyone gander a guess at what the minimum signal might be to have this work?  dBm or µV would be nice to know.  I had one search produce a result of -70dbm which seems very strong, but I think they were talking about WiFi so in the GHz range.  What I'm trying to do is calibrate (somewhat) Radio Mobile to what it's showing me and what I'm actually receiving.  I know I'm not at the minimum because communication is fair to good, not sproratic or choppy.

Anyone want to gander an educated guess?