Centering table in an existing pit

When it is all figured out, I hope someone will post a detailed explanation of how to properly center a table in an existing pit. Installing a table in a new pit is fairly easy - dig a big hole, install the center bearing and set the bridge on it. You can actually use the table itself as a big compass to lay out the pit walls. But having run a turntable myself for 20 years, I know how close the clearances are - a table that is 1/2" off center is likely to bind on one side. As I'm somewhat involved in another roundhouse restoration project, this is information is relevant.