But these are still numbers plucked from the air (or as you put it, from the 'future'). I want *tangible, material bases* to start from if any.

Another far more sensible model I've found is slicing pie. Each founder's input % of the pie pre-'bake' is their % of the rewards. And what makes up for one's slice of the pie? The dollar-value you would've earned if you worked somewhere else, times the period of baking. These can be tweaked accordingly to the type of investment put in. IMO, it seems far more grounded compared to say a flat 10%.

To quote good old Chester Karass (https://www.amazon.com/Business-Life-Dont-Deserve-Negotiate/...): you get what you negotiate. In an ideal world, people are compensated according to their contributions (and when hired, expected contributions.) This isn't a once-and-done thing, though - compensation (in the form of company ownership) gets adjusted all the time. The flat 10% is just a starting point (a typical CEO level of ownership in a start up, although this number varies quite a lot, due to aforementioned negotiation.) If a CEO screws up badly enough, they might get fired even before their stock vests.