how close can an Imax2000 be mounted above a Maco beam on the flat side
before there is some kind of interference ???????

'Best' distance is all that you can get. If one is in the 'field' of the other, there's going to be some kind of interaction. May not even be noticeable, but it'll be there. The most common quoted separation is usually on the order of 1/4 wave length if possible. Not possible for whatever reason? Use what you can get and still keeping things sort of practical.
- 'Doc