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%.