The starting mark is an important piece in being a jumper because it is a starting off point to help you best gauge a consistent distance from the board; and possibly help you to avoid scratches. I put this last in the "basics section" for a reason. Read more below.
Don't find one right away!
If an athlete doesn't know his/her starting mark make that a very low priority at the beginning of the season! Too often coaches find an athlete's mark too soon because they have a desire to get that down on paper ASAP. Or, many times there is an early season meet they must prepare for (which is a whole other topic on its own).
By doing this, a coach is putting too much stress on the mark itself and will inadvertently make his/her athlete place a bigger focus on scratching than is necessary. I cannot tell you how often younger athletes obsess about scratching and have poor jumping experiences because of it. Make this a "wait and see" focus and your athletes will follow your lead. There is no reason to have a mark if you doubt your athlete can hit it frequently and consistently. If you give it to them too early in the season you are setting yourself and your athlete up for failure. Once they've proven that they can start and sprint properly they are ready for a mark!
The "Backwards Approach"
Now that your athlete is ready to get a mark the next step is to do a backwards approach to assign them a starting number. I recommend that younger your athlete is the less total number of steps you assign. For freshmen athletes I have them do no more than 11-12 total steps. I do this because less is more for inexperienced athletes. Giving them too many steps would only give them more area to experience difficulties (thus increasing their chances of scratching). For sophomores I assign 13-14, and for juniors-to-seniors I assign 15+. One thing you'll need to keep an eye on is whether an athlete needs more distance due to a slower time accelerating (because you want your athlete at their top controllable speed). This is a case-by-case basis so make sure to communicate often with your athletes to check their comfort/thoughts.
Information you need prior to doing the backwards approach is as follows: Which way your athlete is most comfortable starting, their jump foot, and their age/experience-level. Once you know these things you will know the exact number of steps your athlete should do in their approach. Here is an example:
A sophomore starts with his/her right foot back in the crouch start and jumps with their left foot (which is forward in the start). This means that the number of steps will be even because their first step will be with their right foot, and their second will be with their jump foot (the left foot). Knowing how they start, knowing their age, and knowing their jump foot leads me to assign them a 14 step approach. This can be confusing, but is easy once you get the hang of it! This will be further explained and shown (for visual understanding) in my upcoming book - stay tuned!
During a season's worth of time an athlete has the potential to change quite a bit. Some may get faster (don't expect too much high school coaches), stronger, better conditioned, and/or mentally stronger. When these changes happen the athlete may or may not have changed his/her ability to perform the approach to a better extent. I spend a great deal of my overall practice time "assessing" marks and working on approaches so that NO jumps are wasted on we are competing. The athletes that work deserve to hit that board every time!