The things you are asking about have been discussed MANY times in these forums, and unfortunately you will not find simple answers to all of your questions. My formal training and experience in the industries of avionics and broadcast radio and television is from the ancient time of analog, but the basic principles of radio signals have not changed:
Signal strength and SNR numbers are RELATIVE numbers, not absolute things that can be compared between two different boxes. But at least they give you a place to start.
On a Bolt, 72% signal strength is the MAXIMUM you will see, since the box has an automatic gain control (AGC) circuit that will actively reduce any stronger signal to prevent problems from overdriving the rest of the system. A Bolt will generally produce a perfect picture with any signal strength over 40% IF, AND ONLY IF, the rest of the signal is "clean".
SNR means signal to noise ratio, and it is an indication of one aspect of how good the signal is. "Noise" comes from many things, and it generally means any part of the signal that is not the desired part. No electronic circuit is ever totally quiet, but the system can compare the base level of steady signal that is ALWAYS there with the variable part of the signal being received to produce an SNR number. By definition, you ALWAYS want a higher SNR ratio; ANY "noise" is bad, so less is better. Unfortunately, the variable part of the signal you are receiving is not always a good part of the signal that you want, so just having a high SNR number is not a guarantee of a good signal either.
It is also important to remember that since the Noise part is generally at a fixed level the Ratio will always go down as the Signal strength drops. With a Bolt, 72% and SNR of 29 dB SHOULD be excellent (ignoring other possible signal quality problems), while 40% and SNR of 16 dB should be acceptable, along with anything in between.
