Finding the center of the work or the 0.00 position, is easier if you have a known sized workpiece centered in the chuck.
Check that the workpiece is centered and measure the diameter.
Next, you want to bring the too tip up to that diameter until it touches, I use a piece of shim stock as a gage. I wiggle the shim stock between the tool tip and the work, while advancing the tool tip until I can feel the shim stock dragging between the two. Now, I know the thickness of my shim stock and I know the diameter of the workpiece. It's just simple math to get me to zero.
I use the same principle on touching tools off it a mill, only I use a 1" gage block and I raise the cutter until the block will slide under the cutter. Never lower the cutter into the block, because if you make a mistake and crank just a bit too far it can break the tip of the cutter off. If you are raising the cutter up, if you make a mistake and go too far, nothing gets broken.