Is there a math equation or ratio for how much overhang a tabletop could have over the base and be stable?

Seems unlikely to be stable. Draw a picture for yourself. Have to know the dimensions of the current table base. Say that the current table has an overhang all around of 6", then the base is 29-12 = 17" by 47-12 = 25". Your new table will have a total overhang of 14" which in the width will be at a ratio of 14:(17+14) = 14:31 over the nearest set of legs in the width. Note that this ratio and the weight of the table are going to determine the stability (the smaller this ratio and the greater the weight of the table the greater the stability). In length this ratio will be 14:(25+14) = 14:39. Suppose the table top has a weight of x pounds per square inch, and each leg weighs y pounds: Then the tipping weight t at the edge of the table in the width direction will be defined by:

14t + (14/2)14(63x) = (31/2)31(63x) + 17(2y)

t = (765/28)x + (17/7)y. Suppose the legs weigh 5 pounds each and the table top weighs 2 ounces per square inch then the table will tip if more than t =~ 27 pounds are placed on the edge. So you can see how it works. If you put a large weight in the center, the table will become more stable since the rhs will be larger. Then you have more to fall when it tips

18 + 0 = 18

As long as you center the table top on your table base, it should be fine. You could put a huge table top with 100" inches all around and (as long as no one bumps it) it shouldn't fall off.

That's because the center of mass of the top is at the center of your table base.