The FTM-100 S-RF indicator has no scale, which makes it useless as is. I can't find any good reason for Yaesu to omit giving any information about that scale, even on the user's manual.
S-RF readings vary from one transceiver's model to another, that's a known fact, but I truly believe that Yaesu made many steps further with this absurd meter thing.
So, for all intents and purposes:
Equivalent S-meter scale source: Wikipedia