A laser, used similarly to how many people use them to check wall thickness while hollowing, can be very helpful in setting depth of cut for Oneway cores. No math or spacers needed, at least not when coring from the outside in (making the biggest bowl first).
I reused the laser that came with my Gizmo hollowing rig. Got a friend to mill a steel rod so one end fit in the hole in the Oneway base that normally holds the coring knife. The other end of the rod fits the bracket that holds the laser arm. I put the 11.5" coring knife in the Oneway base and moved it so that knife just touched the spindle on the headstock. Then locked down the base. Then removed the knife and installed the laser arm. Then adjusted the laser arm until the laser beam just touched the spindle and marked that position on the horizontal arm that holds the laser. Basically, this calibrated the laser arm so it would show where the bottom of the cut is with the 11.5" knife. Repeated the process with the other knives so now my laser arm is calibrated for each of the knives.
To use it, mount your blank on the lathe. Decide what knife you want to start with. Set the laser arm for that knife. Shine the laser on your blank and move the base around until the laser shows the thickness/position of cut you want on the sides and bottom. Lock down the base. Install the knife and go to town.
I go one step further for bottom thickness, especially when trying to go thin. I measured the distance from the head stock to to the outside face of the jaws for each of my chucks. So, for example, it's 4-3/8" from the head stock to the face of the 4" dovetail jaws on my Nova SN2. Say I want 5/8" bottom thickness. I move the base so the laser beam is 5" from the head stock on the center line of the lathe bed and lock it down. This means the bottom of the cut (inside surface of the largest bowl) will be 5" from the head stock. 5" inside minus 4 3/8" to chuck jaw face (outside of bowl) = 5/8" bottom thickness. I've been able to get very reliable thicknesses, +/- 1/8", using this method. I use 6" combination square held against the head stock to do the measurements when setting the base position.