I owned a Corbin C tool, but never used it,no instruction were included. A nearby commercial reloader did a big batch of 45-70 that needed a cannelure because of recoil. He placed the nose of a loaded round against the stop, then using a loose projectile as a guide, adjusted the stop out until the tool would indent the outside of the case just below the base of the bullet. I believe Corben intendded that the Cannelure be placed on the bullet for the case mouth to crimp into.
If this isn't enough info, try to contacting the manufacturer.
I try to adjust the sizing of the case mouth to have 3 to 5 .001" of neck tention on high recoil loads, or if it is a black powder round, the powder column supports the bullet during recoil. hope this helps,Ivan the Butcher