Then comes the hard part.
Once you get past the theoreticals, you deal with things like multi-stage calculations, tank weights and materials, propellant mix ratio issues, drag issues from vehicle size, development costs of the technology, development times, propellant costs and handling issues, modifying ideal trajectories for things like Max Q, and dozens of other things that pull you away from the "ideal" numbers.
Somewhere in the middle of that cloud of multiple variables is a workable and cost-effective design.