Reply by ChuckM

In my opinion, you don’t need a dead flat surface for the whole top. The critical area is the area where you put the fence and the router. If that section and the surrounding area are flat, you’ll be fine in most cases. The MDF is flat but not necessarily so over its entire surface and the same goes for the work pieces (large) that you’ll work with. Even if your router top is totally flat, other factors such as the fence setting, the operator’s skill, stock preparation (true & square) etc.may affect more the outcome of your cuts. So don’t lose any sleep if the router tabletop isn’t 100% flat. Trust your eye and hands … if it looks flat, if it feels flat, that’s flat enough.

-- The time I enjoy wasting is not time wasted