Yes, you would put backing on the reel before the leadcore. A couple of things to remember on lead core. First, if you are going to fish structure (cranking on the bottom), then I like to use reels full of 10 colors of leadcore. This will take a 4000 series reel. I like to put on 200 feet of 10 pound Berkley FireLine for my backing. The thin diameter allows me to put on more line but 10 pound is very strong so I don’t have to worry about it breaking.
For open water cranking, you can use lead core to get cranks down deeper than they can dive on mono or FireLine. What you have to do is figure out how much lead you need for your situation. A rule of thumb is 1 color gets you 5 feet at 2 miles per hour.
Some examples. If you are fishing a lake with a max depth of 20 feet – then 2 colors of lead is often enough because even a stick bait dives about 8 feet – add 10 more feet (for the 2 colors) and you are close to the bottom. Slow down a little (1.5 mph) and you will be banging bottom.
If you are fishing 40 feet of water in the summer, most of the higher action baits can get about 20 feet deep. So you need 20 more feet from the lead – use 4 or 5 colors.
If you are fishing 35 feet but will be slow cranking in the fall. 3 colors of lead will get a stick bait close to the bottom at 1 to 1.2 mph.
So look at your depth and the speed you will be fishing to try to determine a good number of colors to splice in.
For this open water fishing I typically use a 50 foot leader of Berkley 10 pound XT.
I do like (and use) the two reel method for determining the amount of backing. For spliced lead I typically go with 10XT so that I can easily attach to an Offshore Planer board. Remember also, that if you splice in 3 colors of lead and it isn’t quite enough to get to the depth you want, you can let out some backing and the system will keep going deeper.