sdr - Coming up with a squelch value

I am building a scanning-type system. I have samples in $\textrm{dB}$ and I want to determine when to allow them through. I am trying to decide between allowing them through when they reach a percentage over their normal value (or of the noise floor), or if I should use a static $\textrm{dB}$ value. An example of the first way would be to let a sample through when it is $50\%$ over the normal value. An example of the second would be to let it through when it is one $\textrm{dB}$ over the floor.

Which way is standard, and what are standard values I might use? I understand it depends on the application, but I'm just trying to get a rough idea.