In pre-FMC aircraft, we needed good rules of thumb to "ball park it" before we were able to set up more accurate numbers.
We still need them in FMC aircraft for "reasonableness" checks of the FMC information.
The GIGO (Garbage In = Garbage Out) concept applies not only to user data inputs, but also to those programmed within the FMC, particularly evident after installation of a new Nav Data Base.
Sorry to go "Off Topic" RichT, but the thread was starting to go a bit off topic all by itself. I hope you gain many more of the responses sought in your original post.
Regards,
Old Smokey