Yes you can add more flavor if you think it's weak on the flavoring. I think it's always better to start on the low end and add more if needed. As for how much more to steep I'm the wrong person to ask, but here goes. I think steeping is really overated for the most part here. My mixes are usually good to go from the start (shake and vape). Not saying some don't improve marginally tank after tank. If they do, I consider that a bonus.
I constantly read people say their mixes are unvapable when they make them and then magically turn great after steeping, cooking in a crockpot, vibrated in a UC, set under a full moon at high tide, etc

Not sure what their doing to their mixes, but I'm glad I'm not doing it to mine

. I've made a few unvapable and everyone was still bad a week later, month later, and I just recently tried a few that are 6 months old. Guess what I still don't like them. I've found the only magic is changing the recipe when I make something that I think is unvapable after mixing. Good luck!
Cheers,
Steve
Edit Before anyone flips out, there are always exceptions. For instance, I make a cheesecake recipe that I think is better after a few days, but I don't think it's unvapable after mixing.