Ian,
It is more important that your lathe is level and without twist rather than the mill.
The most important part, as Jason stated is the tram between column and table.
The main reason for levelling the mill is to reduce moving strain, and thus, expected wear on the bedways. As I have a very heavy table on mine, that is why I went to such lengths, but even now it isn't perfectly level, now it has settled, somewhere around 0.003" over the 36" table length. To me, it isn't far enough out to justify me going to the effort to get it spot on. Quintuple that figure, and I personally, would start to worry about levelling it up again.
If I was you, I would concentrate more on getting the level only near enough (one or two marks out on the bubble), but still making sure that the base is rock solid, then concentrate on getting your column perfectly in tram with the table, both front to back and side to side, then your vice jaws completely parallel to the table and square to the quill in all directions.
If you get those bits done, you should find you will be able to machine to very good standards.
John