Bike fit / correct riding position

For a healthy body its not.

3mm can be the gap between life and death regardless of being healthy or not, I had the saddle set a little too far back previously when changing saddle, and was wondering why it felt hard to pedals and had a sore back, 4mm forward it's sorted.

3mm is the difference between agony and bliss? Bloody hell, I wouldn't have thought the body could be that sensitive.

Probably not on a commute of five miles, but the difference can be night and day after several hours on the bicycle, the smallest issues can become a major problem.