Achieve alludes to the absolute length of a fighter's arms, estimated from right fingertip to left fingertip when the arms are parallel to the ground — consider it your wingspan. A safe distance estimates only one arm, shoulder to clench hand.
Sonny Liston was 6ft with a whopping 84'' reach. He may only be a Super-Bantamweight, but Carl Frampton's 62" reach is very short, even for someone that stands at just 5' 5".