Flat or slightly concave seems to be the best way to go.  I usually 
have been trying to leave them flat, but if I think that it may be 
slightly convex (a high spot), I sometimes try sanding the belly down 
some with some sandpaper under my finger.  I leave the outside frame 
of the table as is.

I think sanding the table on glass in an "X" pattern is best for 
cuting down a table.  I then finish with lengthwise sanding.  Usually 
slowly with heavy force down to keep from rocking the piece.

Any other table cutting opinions?

Thanks for the measurements on the Link. The tip area seems a little 
strange indeed, but I have had two original slant signature soprano 
mpcs that seemed to have this same tendency. I have the measurements 
somewhere, but both didn't play very well, at least for me. When I 
have some time I'll post some measurements I have collected. As I am 
in Europe (Netherlands) I have feelers in mm, so you would have 
to 'transpose'.

When a table is very uneven - e.g. as a result of earlier attempts to 
reface - I usually do the same thing. I make a 'cross' or a concave 
in the table, so that it rests only on the four corners of the table. 
Then it's easier to sand it down to a flat table again. Sometimes I 
don't sand it down all the way. After that the rails usually have to 
be adjusted.

BTW I am not very good in math. Some time ago I made a kind of sheet 
just like you published in the 'files'. I remember vaguely how I got 
there, using mostly common sense. You seem to have a quick way of 
calculating 'arc' curves. Could you please explain this to me, or 
tell me where to look?