Recessed lighting is fairly directional unless the bulb extends below the surface of the ceiling. A general rule of thumb I use is that the light circle is equal to the distance the light is away from the surface to illuminate. In other words, if you shine the light down from an eight foot ceiling to the floor, the light circle diameter will be about eight feet across.
Since most tables are approximately 30-32 inches high, you will have a light circle diameter about 64-66 inches wide with an eight foot ceiling. If you overlap these diameters, you can eliminate all shadows.
Best,
Mike
I think we can actually do the math all the way here. If you want to have one row of lights down the center of the long dimension of the table, you want to make sure to use only the portion of the light circle that is at least 54" wide, so the light reaches from side rail to side rail. If you fit a rectangle inside the light circle such that two sides of the rectangle are parallel to the end rails and 54" long, how long are the other two sides? That will tell you how far apart, at a maximum, you should put the lights.
Using a 65" diameter as an example, the unknown length of the rectangle calculates to about 36.2". Thus, you can put one light in the center, and one 36" away on each side and cover the table with three lights.
You should measure the distance from your ceiling to your table, then do the math for that diameter.
One thing to remember is that you're not eliminating shadows with the lights above the table. If you don't place your lights so that they illuminate the entire surface, yes you will have dark spots, but these spots are not shadows. Shadows are caused by objects (like the rails, or other light fixtures) blocking light from sources other than your table lights. For example, when a bunch of tables are set up in a ballroom like in the casinos for big tournaments, each table light will cast a shadow on the next table unless the ballroom lighting is dimmed. That's why the room is darkened while the tables are in use.
So, while you can measure and calculate what's required to cover the surface of the table with light, you can only remove all shadows by eliminating other light sources that may significantly illuminate your table.