If you're using ONLY crystal malt and other already converted malts, you can use whatever volume you choose, probably without any negative consequences.
However, if you've got any other malts like Munich malt, victory malt, two-row, etc, you really want to still with the 1.5 quarts per pound of grain so that you can convert the malts, avoid a too-high mash pH, and avoid tannin extraction.
In my mind, it's just easier to always plan on using 1.5 quarts of water per pounds of grains at 150-155 and you're always 100% safe.
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