With today's existing banking products, it is indeed basically impossible to keep 0% in cash, because you then couldn't pay your rent etc.
On the question of how much money you should keep in cash, I think it really depends on how much savings you have. Most experts recommend a rainy day/emergency fund worth 3-6 months of normal expenses. If you invest that rainy day/emergency funds in a Financial Choice account, it is important to choose an investment strategy with an appropriate risk/reward profile, and adjust the savings target to account for the additional risk from the investments. Betterment has a pretty good writeup on this idea https://www.betterment.com/resources/safety-net-funds-why-tr....