For 5 gal in fermenter batches that will yield 6.5% ABV you will need 10 gal of ~1.060 OG wort. Assuming a low boil-off of 1 gal per 5 gal batch you will need 12 gal of wort at 1 + 0.060 * 10 / 12 = 1.050 pre-boil SG. That would require 6 gal @ 1.100 pre-boil, or 5 gal @ 1.120 pre-boil.
Assuming you routinely get 100% conversion efficiency, and a consistent grain absorption rate of 0.07 gal/lb, then for:
6 gal @ 1.100 SG pre-boil, then numbers look like this:
Grain = 26.7 lb
Strike Vol = 7.87 gal
Mash Vol = 10.0 gal
Mash Thickness = 1.18 qt/lb
Mash Efficiency = 63.6%
5 gal @ 1.120 SG pre-boil, then number look like this:
Grain = 30.26 lb
Strike Vol = 7.12 gal
Mash Vol = 9.5 gal
Mash Thickness = 0.94 qt/lb
Mash Efficiency = 56.3%
Your task won't be easy. If you get less than 100% conversion efficiency, or higher than 0.07 gal/lb grain absorption, then the numbers will be even worse.
Calculations done with
my mash and lauter simulator.
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