Wondering if someone with good experience on a CL604 could tell me the airplane's favourite altitude to cruise at? (perf/fuel burn ratio)
Not talking about LRC flight over the Atlantic, rather around 2 hours air time? and 4hrs?
Depends on weight, but we always go to 410 (max operating ALT) if we can and if the leg isn't too short.
For a 2 hour trip with normal fuel load, 400/410 would be best. If you're tankering a bunch of fuel, you may be too heavy and have settle with 380/390 or less depending on your load.
Good rule of thumb with the 604/605 is "39 to 39". Meaning need to be at a gross weight of 39000 lbs to be able to cruise at FL390. At 43000 lbs (approximate max fuel load for 2 hour trip and be able make landing wt) you'd probably only be able to make 360/370 initially.
On a 4 hour leg, you will probably almost always start around 380/390 then climb to 400/410 when weight/OAT permits. If you are tankering fuel, it will be lower.
Of course all this also depends on OAT at altitude.
So as you can see, not a simple answer, unlike some smaller jets.