A while back did a lot of work figuring out proper RF signal levels to feed the 4th gen Tablo. Here’s a link that summarizes most of it …
Make sure to pay attention to the signal level units. Also, the Tablo internal amp is easily overdriven. Turn it off unless you absolutely need it. The Tablo tuners themselves are ATSC 1.0 spec, so they can handle a pretty big range of signal levels per that spec before they are overdriven. I have verified this through my testing.
One more thing … s/n ratio is way more important than absolute signal strength for reliable signal decoding. Look for s/n ratios above 30dB for really solid decoding. Of course, 20-25dB may allow signal lock by most tuners, but it won’t necessarily be reliable. Anything less than that will be crappy if it works at all.