How spot on is the dot of GPS in iPhone 11?

Read about it here -

https://www.gps.gov/systems/gps/performance/accuracy/

The quick answer from this page is approx. 16 feet but many factors can change this.

As long as you have an Internet connection, wi-fi is turned on, and are not inside of a building that will block reception, you should be good. I have an iPhone 13 Pro, and mine is quite close most all of the time. You can take it outside the house/building when it is visible to the open sky to test and it should help you determine how close you believe it is.