Nope! no rule of thumb, at least not yet.
I like to think in terms of relative value. Do you want a Bitcoin or a Doge? Pick a number between 1 and 99 - If you want something that is very valuable, rare and traded fractionally when it matures (the easy money days are over) than stick 6 zeros behind it. (xx,000,000)
If you want to Doge it up and make people generous, stick 9 zeros behind it (xx,000,000,000)
For LTB we wanted something in the middle so picked a number (51) and stuck seven zeros behind it (510,000,000)
When thinking about who should earn the tokens and how many, look at your ecosystem and determine which groups and actions, if they were not there or were not performed, would render the platform worthless. In the case of LTB there are three groups of decending importance. Content Creators, the Audience that consumes that content, and the platform that enables both.
Do you have a particular project?