Your voltage readings are a nominal 18VAC. However, the real pudding is the voltage drop under a real load. At this stage, it isn't worth getting that pedantic over if you are happy with the trains operation. Go test run your most power hungry train and start running trains!
To directly answer your questions, you can always add more feeds. The only detriment will be what others think of you when you have a feed every inch of track.... A more powerful transformer doesn't automatically necessitate adding more feeds to a properly designed and executed wiring plan.
Now remember the command to run trains? If for some reason you have power related issues, come back to the forum here and let us know what is up and I'm sure we will get you straightened out. You are off to a fantastic start.
