Some will go as high as 20% water but it needs to be warm weather and the bees must be able to take it fast for processing because you run the risk of fermentation. 10% is good for preventing granulation of the HFCS and you don't have to worry about fermentation.
My HFCS Type 55 is 24% water upon delivery and I diluted it 10% with water.
I have about 50 gallons left over from last November and it's stored well.
The more that you dilute the syrup the less the bees will like it and it may sour or grow black fungus.
You might want to test your dilution ratio in small batches
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