I suggest you don't read too much into it as the “very good” expression depends a lot on the personalities of your interviewers. Some use it extensively as a general figure of speech to say “we're done”, while others only use it when they want to give praise.
In the context of your interview, your interviewers usually want to see you display the core casing skills - which include problem-solving, maths, communication, insight generation, among others. You need to display a good balance of these without any red flags to move forward.
On the math part that didn't go smoothly, as long as you showed good active listening and you reacted well to new input, you should be fine. Mentioning proactively the “so what” is definitely the right thing to do as it shows you're driving the thinking - well done!
Most important is to ask for feedback once you receive an update on your status - whether it's a move-forward or a rejection. Either way, doing so will help you learn the weaknesses/areas for development that you need to address for your upcoming interviews.
Hope this helps!