I've always (with good good results) prepped 10 gallons of water for my 5.5-5.75 gallon batches and calculated the final mineral content based on my final fermenter volume...something like this
...7.59 (pre-boil) / 5.5 (fermenter) * 58 ppm CL (10 gallons treated water) = 80.04 ppm CL...