Q.mature fruit bearing grapefruit tree
I bought my house and I know very little about it. My question is in digging a water well, how far from the trunk? I know that it should be as far as the canopy but is to be started from the trunk the distance from the trunk to tree canopy is about 6 ft?

I believe if you stay outside the canopy of the tree, you should be safe.
If you have the ability to choose your well location, I would most certainly want to keep established fruit trees a safe distance from any possible damage.
From personal experience, we were able to have our well drilled approximately 10 feet away from a large established tree canopy.