We theoretically studied the sergeants-and-soldiers principle of chirality amplification in self-assembled, supramolecular helical polymers and show that a mapping is possible on a two-component Ising chain, in which one of the components has a spin of fixed orientation. The relative abundance of this component is regulated by a chemical potential. Our theory quantitatively describes recently published measurements of the optical activity of linear assemblies of certain discotic molecules in solution. We find that the strength of the chirality amplification depends strongly on the strength of the coupling between the spins, which translates into a free-energy penalty put on a helix reversal along the backbone of the self-assembled polymer.