Formula

10% cement, 75% aggregate, 15% water [1]
150 kg/m3 of cement - with concrete at 4000 lb/cu yd [2] - was sufficient to get about 6000 psi concrete strength [3]. Does this mean abouut 8% of cement?
500 lb per yard of cement in concrete is recommended - read a thesis on it - [4]

Concrete Slump Testu

Measures inches of collapse, from 0-12", of a wad of concrete in a cone. 12" means it's complete water. 4" is very stiff.