lead core question

I can see when trolling a light lure such as a worm harness on lead core line that it would be easy to hit a target depth simply by marking the colors as they leave the real.  Target depth 40 ft,  8 colors out.  But what about a deep diver such as an 800 series Reef Runner that has the ability to reach 22 ft without lead core?  If I want my Reef Runner to hit 40 ft,  do I just let out 4 colors of lead core (drops 5 ft. per color) and assume the diving ability of the lure itself will take it down the rest of the way?  I'm really confused on this as I'm brand new to lead core this week.  One other question I have is if I want to hit a target depth of 40 ft. and I have to let out 8 colors to do so, do I actually let out a bit of the 9th color to make sure all 8 colors that I need are actually submerged? Thanks and any other helpful lead core tips would be greatly appreciated.

I agree for the most part with drumR.  The leadcore and crank will cancel each other out a little bit.  I've seen some depth probe readings of cranks on lc and it appears that you'll get all of the lc depth plus the diver depth from the leader length and then a bit more.  For example, if you have 5 colors with a 50' leader, you'd get the 25' depth from the lc plus the 12' for the leader plus an additional foot or two...this approximates what the probe chart showed, although I'm sure the probe created a bit of resistance.  If I remember correctly the tests were conducted at 2 mph, which is important because lc is extremely speed sensitive.