You know what? It really doesn’t matter. Just keep on going with your ribbing, when you have the length that you want then start binding off at one of the sides. It won’t make an ounce of different which side or if you are ‘truly’ at the beginning of the round. It’s a bit different anyways cause now you’re essentially working top down when before you worked bottom up.
A tip for you though, when it comes to binding off…try to bind off loosely, if possible bind off with a needle one or two sizes larger than what you used for the ribbing. The last thing you want is a bind off that’s too tight!