May be a bit repetitive and requires a modicum of patience, but it's always worked for me. I currently have six (ETAs 2836-2s & 2671-2s, and Asian 7750s & 7753s) all synched with each other, almost to the exact second.
The way I do mine is as follows:
1) Open the digital clock on your computer.
2) Remove the case-back
3) Stop the second hand on the watch by pulling the crown out to the time-set position, to activate the hacking movement (stop the second hand directly at the twelve o'clock position).
4) Then roll the minute hand forward to where it will be, one minute from now.
5) When the digital second hand on the computer reaches 12 o'clock, push the crown back in so the watch & the computer's clock are now synched to the exact hour, minute & second.
6) Put the watch on a winder for a night & compare your watch to the computer's clock after 24 hours to see how far off they are.
7) Pop the caseback. For example, if the watch is now fast by 10 seconds per 24 hours, simply turn the adjustment screw (C, pictured below) so that it loses 10 seconds for every 24 hours; alternatively, do the opposite, if it's slow by 10 seconds.
(If adjusting via the screw doesn't regulate it enough, on an ETA movement--like the one pictured below--the farther apart you move arms A & B from each other, the longer the stroke, so the slower the watch will run. However, a little goes a long way with this, so use cautiously)
ETA 2671-2:
ETA 2836-2
Each notch towards "+" represents a 5 second gain per 24 hours. Each notch towards "-" represents a 5 second loss per 24 hours. Adjust accordingly
NOTE: For Asian chrono movements, they're slightly different. See the little regulation arm circled in red?:
If it's running too fast, you'll want to move it to the right (counter-clockwise, as shown with the red arrow below) :
If it's running too slow, you'll want to move it to the left (clockwise & opposite of the red arrow's direction).
The regulation arm on the Asian 7750s & 7753s (sub-dials at 6, 9, & 12 o'clock, or at 3,6, and 9) are very sensitive--they only need to me moved a fraction of an inch at a time; not as much as the picture shows (it's merely an exaggeration to illustrate a point).
8) Now repeat steps 3-7, to test it's accuracy. This should be perfected after a couple of days.
I technically have no way to measure the movement's beat-rate via electronics or machinery, but at this point don't really need or care about that, as long as the watch is keeping the best time it can.
I've now got six synched automatic watches that are so precise, I can literally watch the second hand roll over the 12 o''clock position at the very second when the time on my cell phone (or computer) changes minutes. They're damn near almost exact as digital or quartz watches now.
Note: Depending on what position you store your automatic watch in, will play a role in determining how many seconds it gains or loses every night--this is why I keep them on a winder, so that they are constantly moving and not at rest (in the same position).
1) If you lay an automatic watch flat & level (with the crystal facing up), it will usually
gain a couple of seconds per night.
2) If you lay the watch on its side (with the crown facing down) it will
lose a second or two per night.
3) Contrarily, if it's stored on its side (crown facing up), it will generally
lose more than 2 seconds--or more--per night.
EDIT: Chances are, if you've performed all of this and your movement is
still too slow or too fast, it's running dry & will probably need a full service. On the bright side, once done, you won't have to touch it again for another five years.
Hope this is of some help to you. Have fun!
