Yes, that's one way of figuring it out, so you did well with the puzzle. When Emerson tells you what locker you tried to open, you can reason yourself to what the different parts of the key tag code means - like you did!

The puzzle tag code broken up - with the solution 3B12 as example:
First figure (3): The locker segment which your locker is - start counting from the door and go counter clockwise (the arrow circle symbol is the hint)
The letter (B): The row which your locker is in - T means top row, B means bottom row
Second figure (12): Which locker in the row your locker is - start counting from right to left (this is what throws people off since it feels strange, and Emerson gives some hints about it, although I realize they can be a bit subtle)

So - the correct locker is in segment 3 (counting from door counter clockwise), bottom row all the way to the left.

And for anyone else who wonders: No, how fast you solve the puzzle does not affect what Emerson thinks of you - he will merely tease you a bit for not figuring it out and finding and opening your locker for you. Then you get to tease Emerson back which I think is fun.