What's the point to use for calculating torque?

Hey there, I know I'm brand new here, but I've got a question that I'm trying to figure out. I'm not an engineer of any sort btw, just a humble millwright ;) So anyways, I'm working on a bit of a project in my head right now, and I need to know what distance to use when calculating torque. I have a 8 inch nut that needs to be tightened down on a shaft. Would I calculate my torque off of the OD of the nut, or would I measure off of the center of the nut? Any help would be muchly appreciated! Thanks a lot!

Austin