I find that as long as I heat the water and make sure that the sugar is all dissolved the 2kg/1L ratio works well for me and does not settle out.
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Please have a look at
http://sugartech.co.za/solubility/
Stick in some likely temperatures to be encountered overnight in your feeder.
2kg + 1 litre (=Kg) of water is 66.66 (recurring) %.
At 60C (hot water) it will hold 75% (so 66% dissolves quickly)
But at 10C water will only carry 65% sucrose, so 66% will try to crystallise out of solution ... or no matter how long you stir it at 10C, it won't all dissolve.
Of course, if you are measuring the sugar and water to an accuracy of only ±5%, your results will be variable!
The thing is that metric 2:1 is right on the limit of what can dissolve.
If you are trying to make it as strong as it can possibly go, then that is your target. And some people would even add excess and strain it off later - or simply not be bothered by the (wasted) crystallisation in the feeder.
What effect a saturated solution might have on the bees, I don't know - but its not a situation they would encounter "in the wild" - its unnatural, and I can't see that it would have any benefits. Any evaporation before inversion (which is a non-instant process) is going to lead to crystallisation on the bees, which I can't see as being ideally helpful --- unless you were expecting granulated stores and were trying to get there as quickly as possible! .
The difference between 66% (metric 2:1) and 62% (imperial 2:1) isn't huge, but - with accurate measurement - it does take you over the edge and into recrystallisation territory.
