Of course, if you have a refractometer you could measure the current water content, if you don't know it. You would have to know that to know how much water to add. I would assume that HFCS would be about 20% or less water in order to keep it from fermenting. Assuming that (which you may need to measure to be sure of) then it's just a matter of weight or volume. If you have 4:1 syrup (20% moisture) then you need to add enough water to end up with 1:1. So if you have a gallon of 4:1 that's 26 ounces of water and 102 ounces of sugar. You need to end up with 102 ounces of water, so you need to add 76 ounces or so of water to one gallon of syrup to get 1:1. That's 4 - 3/4 pints of water per gallon of syrup.