No, no confusion here! LOL. Just a lot to learn.

Bear with me, this is intended to show how much I either think I know or expose just how much of an idiot I am when it comes to sinking lines.

Cortland LC-13 Lead Core says it has a 8.75 IPS sink rate. 13 grains per foot which would be 390 grains for 30'. Find my spot I fish, find the bottom and cut off enough to just keep it above the bottom or where the fish are suspending. Weighted and unweighted flies will change this sink rate but not very much unless the fly is bulky and light or really heavily weighted, right?

What does that mean? Using all 30' you get 8.75 IPS sink rate? 15' you get 4.375 IPS sink rate? 10' you get 2.92 IPS sink rate?

Coupled with the 3.5-4 IPS sink rate of the Cortland 333+ WF/SIII line.

BTW, I am guessing here so correct me if I am way off base, please!
LLC-13 coupled with Cortland WF/SIII gives the following, theoretically:
30' = 12.25-12.75 IPS
15' = 7.875-8.375 IPS
10' = 6.42-6.92 IPS

Or is that the 13 grains per foot gives you a sink rate 8.75 IPS if you use a foot or use 30'?