A way you can test your timing in practice mode is to set the AI to auto-block and do your b1,2,1. When you dash up and go for your follow-up, you'll get one of three results:
-The first hit of your 2-string will whiff, which means you're swinging too soon.
-The entire string will hit the opponent, which means you're swinging too late.
-The AI will go into block-animation, which means you're timing it correctly. At this point, they can't poke, duck, jump, teleport, armor, or X-Ray because they'll just eat the full combo from the ground.
That's obviously gonna be tougher to time online depending on the latency.