>So *IF* my calculation is correct, I should put 85 litres of water in each
>200 litre barrel and then top them up with HFCS55 when the tanker arrives.
 
I wouldn't do this unless your water and barrels are sterile and you figure
you can use the entire batch in a week or so.  Otherwise, dilute to about
65% so that it does not crystalize and then dilute further when using it.
 
50% syrup has a pretty short shelf life and will ferment quickly in storage.
65% syrup should last a month at least without significant fermentation.
FWIW, we always feed at 67% solids, unless the weather is cool and we have
to add a *little* water at the least minute to get the syrup to flow down
the hose.
 
Allen