Add 8g gypsum to mash water to soften. Target 110 ppm chloride. 55 ppm sulphate
Based on boiling 7.20 gallonsliters at 1.055 for 60 minutes, this will decrease your wort by 1.00 gallonsliters, bringing your after boil OG to 1.063. You wanted an OG of 1.064. It looks like it will be under.