most leadcores whether it be the 18, 27, or 36 lb versions will sink at very similar rates, this is due to the diameter increase and weight increase. These nearly cancel out when you look at the additional fluid drag the extra diameter causes in relation to the additional mass.
You need to also understand how thermoclines and water density changes setup in the water column coupled with speed and currents. If your trolling slower you will see more depth out of the core, if faster less depth. If there is a strong thermocline the density change of the water and movement of the water will cause the core to float in this region and not penetrate to the deeper water. Currents also affect the depth.
Best rule of thumb is 4 to 5 ft per colour, and if you have a thermocline 5 to 10 ft above the deepest your core can reach, your likely not going to penetrate through unless the lines on an inside board on a turn. assuming no current or this is the speed at the ball 1.5-2.0 mph you will see 5 nearly 5.5 ft per colour, 2.5 mph you will see 4.5-5 ft and 3 at most 4 ft per colour.
as for running the core its always best to get all of the core in the water, its nice to have the luxury of having rods setup for exact segments of core and to be able to get the backing into the water also. If you do not have this luxury be sure to get all of the last counted section into the water. You can adjust your depth by adding more or less.
The key to this is repeatability, if your catching fish with so many colours out plus a fraction just repeat it each time, set it let it ride, if no go, adjust until you catch then repeat.
good luck!