You can't go wrong with that approach.
Doing a little calculating, I don't think the 11 rail cars on your layout is so few. If you count the 11 cars off the layout that will replace them during an op session, I get 22 cars. And unless you're going to just switch the same two groups of cars on and off, you could have another 11 cars that you moved off in the previous session, so now you're up to 33. Yes, of course I simplify, and I am making assumptions on how you will operate, and I know you would likely not switch all cars in a session, but if you're operating scheme involves moving cars on and off the layout via an interchange track or yard, then you have to count the cars waiting in the wings to come onstage. That's the point my simple equation is trying to make.
Mike