That may be different for every runner. Have them start when the pitcher begins their stride and see how quickly they're getting off the base. Each runner may have to start at a different point, dependant upon how quick they move. Also, as they become more experienced, runners may have to adjust their start with every different pitcher they face as each pitcher's style can affect their time to release from start of motion. Because of thiis, timing to the pitcher's stride may make more sense than timing to their arm motion. Baserunning is as much of an art as hitting, pitching and fielding. Most good players have adequate base running skills, but the ones that are exceptional at it are fun to watch.
this subject came up before. I'm a believer in the rocker start (lead foot on the bag, runner starts when pitcher's arm is at +/- 12:00)but other coaches insisted their players had better times with a "track start"- back foot on the bag, leaving on release.
for the most part it does match the pitchers stride. I personally prefer to watch the pitchers arm and the stride of the runner match the arm. The reason for this is often a pitcher will step then throw (as incorrect as this is). Runners can get caught off base if they do.