What is slightly confusing (with regard to Direct to and Height constraints on STARS) is when we are given a height restriction which is specified as 'xx' miles before a main VOR. One example is the lorel arrivals for LTN and BHX when, I believe, it is common for "Descend FLxxx 20 miles before MID. Fine-the only problem for the FMC is that the point AVANT is 18 miles before MID and if you try to construct a waypoint as MID/-20 then you get an error msg so you have to build one as AVANT/-2 (ie 20 miles bfore MID). This can complicate the descent slightly at a time when changes to the FMC are supposed to be kept to a minimum (Company requirement) . . I suppose what I am trying to say if that is what you want, the say descend FLxxx by xxx waypoint and NOT expressed as a mileage before another waypoint.