It seems I am beginning to run more steady now and am looking more critically at the actual cuts being made and I find my table is not parallel to the gantry by 3/64 of an inch.  This is from side to side on the short axis.  #hat I am doing is setting Zero at onecorner of the table raising the Z  move to corner 2 set zero and checking it's the same. 

Having determined this small error what is the best material to use to shim the table flat.  I have a solid base with an MDG top with screws holding the mdf down in four places.  This is 1" MDF with T bar slots cut in it and I have T bar in the slots. 

I am not sure if tape or some other material is best to use.  I  am sure others have had this situation, I can see why you really want to protect that table top now it takes a bit of time to level things out for accuracy.

Can you not just surface your top plate?
Hood

Well, I guess I can but wow that would take some time.  I think you mean set Zero on the low corner and begin passed backa nd forth to level the table.  This is 24" x 48" that might take some time.

 a large Flycutter shouldnt take too long but even if it does it will be the best in the long run I think.
Hood

Is that what most do install a table then begin to rout the surface to level it?  I did this backwards then I cut T track inot the top and mounted it.  I would have to be careful not to hit the Alum T track.  

that would save a oot of effort finding the low spots for sure.  As I think about it that really makes a lot of sense.  the you know the entire surface not just corners are perfecly level to the gantry. and full length too.

Would you use like a 3/4 dia bit to do this?  Or is there some sort of fly cutter to use?

I see you already told me that................thanks Hood, I will look for a fly cutter maybe 2" dia or more.  Have to see what I can find I am not removing a lot of material so can be rather large with a 2 1/4 HP router.

Oh a router, only use a flycutter if you can reduce the spindle speed, dont want a flycutter going fast as it is unbalanced and a router spindle is not the strongest and also it will likely shake your gantry quite a bit.
Hood

Sorry Hood but i am going to disagree with you about surfacing the table.

I believe that the machine should be adjusted to get it flat using a flat reference "surface plate springs to mind".

Only when you have it almost perfect should then surfacing be the next step.

The reason being that anything that is substantial in thickness may have that error on the surface. You will also find out what is flat using this method.

Now if you only work on very thin items this advice can be ignored and Hood's method will be sufficient.

Phil