Sensor track location relative to grade crossing

I spent a few minutes trying to search this answer but ultimately I gave up and decided to post.  I am betting this has already been discussed, but here goes.....

I am finally installing my first sensor tracks.  I am installing one on either side of a grade crossing.  How far back have you guys placed the sensor tracks from the crossing?  I have not seen Actions 1 & 2-Crossing Gate Signal Blow but I know I want this action to be triggered.  I want to put the sensor track far enough away from the crossing to give it time for entire sequence before entering the crossing.

thanks for your feedback