10W, It would be rare for a 600nm leg to be flown below 18000'. the Longer legs would be above TA and we would be using FLs exactly as we do now.
My question wasn't 'How often would this happen ?'. It was asking how it is managed when it does, particularly in the US which obviously has a lot of experience of it.
Let's look at another scenario with a lot less than 600NM instead. Leeds to Edinburgh flights for example will initially maybe only get FL180 (and often stay there), or Glasgow to Manchester might ask for FL170 only. Level difference today between Oop North and Oop Further North is about 270 feet if you fly on the airfield QNH's. 270' plus 200' tolerance on each aircraft occupying the assigned altitude is 670' eating in to the expected 1000' separation. As it's part of my job to separate, I want to know how I am expected to do that when the transition altitude is now at a level where aircraft can regularly be encountered. Whether it happens every day or only 35 days a year, I need a safe procedure.
Currently Heathrow holding looses 70 and possibly 80 if the pressure does not allow separation from 6,000', hopefully this problem will be removed by a higher TA
I think the 'losing multiple levels' often quoted is a perception. The Minimum Stack Level is always going to be a Flight Level equivalent which gives at least 1000' separation from the Transition Altitude. In Heathrow's case, it's always going to be a minimum of 7000'. On some low pressure days, because the UK uses whole Flight Levels, then there will be a need to round up the Flight Level to the next one which gives the minimum separation. That can lose you a bit of airspace (up to but not exceeding 1000') and technically (or numerically) one level. But it will never be more than this. Some people think that when the Stack Level pushes up to FL90, then you are losing 2 Stack Levels. This is not the case since FL90 is no where near an actual altitude of 9000' on those days.
Thanks Blockla for an indication of how it is done in Oz. Makes some sense.