hello . I Had some experiences in Surface Traking function .
some time at the start of take off the Desired range finder Goes negative and it cause the copter want to goes down and pilot should put the THR Stick fully up until the Desired goes positive .
it happens some times . you can get it from picture below .
i recommend set the First bias to rangefinder Value just at the take off Moment .
also some time it get positive Bias just after take off .
its a log
it based on Copter 4.5.7 Branch without any Modifying the controls Functions .