I wouldn't worry if I were you. If you had used your mac the other way around - mostly on battery, then battery health would've dropped. From my experience - using Mac on A/C doesn't harm the battery in any noticeable way.
When I got my 13" MBP I would use it on battery (until it'd dropped to 40%, 50%) and then charge to full. I also drained it to calibrate (which was unnecessary) once a month. And that didn't do me any good, quite the opposite. Battery health dropped to 88% - I've been able to get it up to almost 91%, by using it mostly on A/C, but it seems that it won't go any higher. I truly deserve a face palm - because the MBP is less than 6 months with circa 50 cycles and the battery lost 10% of its original juice - all because of my stupidity and reading too many Battery Health FAQs incorrectly

I regret even more because I need to use it "outside"=on battery sometimes and I would like to have all the battery hours that I can get. But what's done is done.
On the other hand I have a 15" MBP - it's almost 1 year old. Bought it used a month ago. It wasn't used too heavily by the previous owner, actually it'd spend a lot of time unused on the desk or plugged in while it was used. It had 93% battery health and around 60 cycles when I got it. It now serves as my desktop and I use it on A/C 99% of the time, I unplug it once a week for 10-15 minutes and let the battery charge drop to around 90%. During that time the battery health increased to 97% and stabilized on that level.