I'v used lead core chasing walleye in the great lakes never for crappie fishing. What I do use is snap weights and you don't need to put as much line out as you do with lead core to reach a desired depths. I use line counter reels for this so I know how much line I have between bait, snapweight and rod or planer boards when I'm using them. I use this also on Stockton in the spring after spawn when they move to deeper water and had good success and picked up some walleyes also. Lead core line is colored in 30' segments and each segment or color will take your lure down another 5' past it's normal diving depth. There is usualle a 20' leader of mono going to the lure then the lead core. If you want to get a shadrap that dives 5' down to 25' you need 140' of line out, 20' mono 120' leadcore. With snap weights i'll let lure out 20' put on 1 1/4 oz snap weight and another 40' of line and I'm at about the same depth as with the lead core. It's been a long time since I used lead core but I think thats the formula to figure out your running depth.
All this is based on a speed of 1 1/2 to 2 mph on a gps. The faster you go the higher the baits ride and the slower the deeper they will go. Give me a call next spring and I'll be glad to show you about snap weight fishing.
Rick
EVERY FISH IS A MEMORY SOME ARE JUST BIGGER