The "Backward Approach"
Generally, the first step in assigning a number is the backward approach. It can be done using other methods, but this specific method seems to be most prevalently used so I'll refer first and foremost in helping to find an initial starting mark.
Before doing this drill coaches need to collaborate with athletes on the front starting foot (refer to the Start page) and how many steps to utilize. The front foot (in my opinion) should be completely up to them. Don't force a belief on them such as "always having the jumping foot forward in the start." Athlete discretion and voice are VERY important!
Once the front foot has been figured out, coaches and athletes need to discuss how many steps (total foot contacts) should be used in the approach. If an athlete is younger or has lesser experience prescribe them a lower number. If an athlete is older and has a good level of experience I would suggest leaving it up to them or assigning them numbers that are a little higher.
Low numbers would be 11-13 steps. Higher numbers would be 15-16 steps.
Beyond that, the only other factor is speed. If an athlete needs a little longer to get up to the desired speed you might need to assign them more steps even if they have a lesser experience. This is something that the coach and athlete will need to (once again) collaborate on.
Here is an example:
A sophomore who has competed before decides to start with his/her right foot back in the crouch start and is a left-footed jumper (which is forward in the start). This means that the overall number of steps will be even. 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! I will go over this in greater detail and will provide numerous videos in my upcoming book.