I agree with Denny that you could simply input the % efficiency into the software to get a recipe that uses less grain.
However, if you feel there’s a correlation between qty of grain and viscosity, maybe open your mill gap a bit to use the same amount of grain but with less efficiency. I personally like ~75% BH efficiency.
If you don’t use software, measure the gravity in the boiling pot and multiply the points by the number of gallons you collected. Then divide by the number of pounds of grain you used. The result should be somewhere around 30.
Often folks complain about not hitting a certain OG (coming up short) and the solution is often tighten the mill gap and/or add grain. In this case the complaint is the opposite so the solution is the opposite.
I use a JSP MaltMill and have placed some match marks on the case to show me where various settings are that leave crushed grain on top of a No14 sieve. With all else being equal (pH, type of grain, etc), the wider the gap = the more on top of the sieve = the less efficiency. (Currently set to 60% on top/ 40% in the tray):
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