Regulation varies across the 5 usual positions so a watch will be set up to get the best average across those.
Trouble is it gives each position an equal amount of time and therefore weighting toward the mean rate, and we don't spend equal amounts of time with our wrist in each of those positions so will always get a different result when worn from that achieved on the machine. So, someone who spends significant time driving will get a different rate from someone who is just standing or walking most of the day.
Set the time in your normal fashion, wear the watch for a day and check the error. If you tell whoever adjusts the watch the amount the watch is out, fast or slow, they can quickly adjust it from that information to get it close to your wear pattern.