I'm not so sure a good answer could be given for your interest in this. For most applications, PicoScope will work well with the 10,000:1 ratio. It would be up to the operator to find the area around the magnetic field that works best. Field strength differ from each application, so the operator must find a consistent spot to compare each reading. There are some COP ignition systems that are shielded so well, it will not give a reading. These are gaining much more popularity, but that shouldn't discourage you because there are a huge amount of these out it does work on. Myself, I really like it. Quick information with practice. Works on coil in cap (CIC) too!
To answer your question though, I think you can trust the 10,000:1 figure for attenuation.