Can't get timing dialed back to tdc

I think the mount has two slots and bolts, one on block, the other on bottom of distributor. The one on the bottom is out of sight. The other option is pull distributor and change the drive gear index by a tooth.

What would have caused this? Someone pulling the distributor and then not aligning it correctly? About how far is "a tooth"? I looked online and the gear drive is spiraled.