It is necessary to get all the high spots stoned down until you have a planar surface. Disregard the divots because they won't interfere with establishing a true planar mounting surface.
An impact ding will not only create a depression/divot, but will also displace some material to leave a raised area/ring adjacent. A drill point can do the same thing. All those high areas have to be removed.
After stoning the high spots down, if the table surface is so pockmarked it's too hard to read an indicator, you'll want to use a surrogate surface to run the indicator contact point on. Any precision flat and parallel piece will do. A smaller piece will have to be moved around, along with the indicator contact point. Two wide, long parallels or anything(s) known to be flat and parallel and will span the tram area will be easier.
Everybody has given the same basic advice. Each of us just says it a little differently.