Everything is hard to say without seeing you. But here are my general thoughts,
For the sub elite guys, approach distance should be determined by body control and the ability to transfer horizontal speed to vertical lift over simply how long it takes you to get to top speed…
Whether you should go from 14 or 18 is also dependant of how you are planning on running your approach. a 14 stride all out sprint might be the same speed at the board as a sub max build up 18 stride approach.
The best way to determine the best distance for you is during technical sessions after the GPP is complete. You should be long jumping every week and starting from 6 strides. These 3 factors should be performed before moving back in distance:
1) The athlete needs to achieve visible lift
2) The athlete needs to be able to control flight and achieve a good landing
3) There should be an increase of about 10-15cm per every stride added…
Example, if after a while you jump 6.50m from 12 and then consistantly jump 6.50m from 14 strides the odds are you’ve found your approacg distance within a 2-4 strides. However, typically what you can achieve from a number of strides you can achieve better from upto 4 strides longer in competition. So if you are hitting good 12 stride jumps in practice you should be able to jump further with good technique from 16 strides in a competition.
Hope that helps.