The requirement is a fixed, constant current, maintaining 10+V across the cell. However you choose to achieve this goal is up to you and there are a whole bunch of ways to do it, all acceptable.
Since the cell voltage remains relatively fixed over the course of the run (usually doesn't fluctuate much from what you choose to set it to), more input voltage to the limiter than you need to maintain a good "safe area" so the limiter cannot ever crap out due to too low a voltage across it is just wasted as heat in the limiter.
Just like in real life, given your operating conditions, things will be different. Either you just go for 30V (or close) and call it even or you do a little measuring to figure out given your conditions what you can get away with and then add 3-5V to that to be safe.