H.Finn - Sorry your Very Senior Controller is wrong and you should tell him so. (Not surprising as most "Very Senior" controllers don't hold validations!)
I just checked the very latest Manual of ATC Services and the word "behind" is not used in any ATC instruction. For a conditional clearance the correct phrase is "After the departing..."
JW411 - I can only assume that you and your colleagues have some special dispensation at your unit, but be aware that saying "behind" is mega-dangerous!