Placement of initial alteration line for crotch adjustments.

 I watched the pants alteration video but, I have problem trying to figure out exactly how many inches down from the waist of the original drafted pants pattern that the inital alteration line should be drawn. according to the pants alteration video, this line appeared to be a random palcement. I just want to figure out the best way to alter the crotch length. Any sugestions. I was just concerned that if the line is not drawn on the same location on front & back pattern, then the peices will not match once sewn together.