I have a recipe in front of me that calls for 1 pound, 2.5 ounces crushed 75L crystal malt. My store only sells in increments of 20L (so in this case, I have 60L and 80L). How would I calculate how much 60L and 80L crystal malt I would need to make 1 pound, 2.5 ounces of 75L?
Honestly, there's often more than 5ºL variation between batches of crystal by the same maltster, so I'd probably just buy the 80L and be done with it.
But, if you want to try to be more precise, the Morey equation for SRM (which is what most software uses) just assumes a linear proportional effect. So, you'd want 3 parts 80ºL to 1 part 60ºL.
In other words: 4.625oz 60ºL and 13.875oz 80ºL.
It doesn't really work that way. You can't proportion out a crystal malt and hope to get a different one.
You can play with math all you want with this. But if you put half a pound of C120 in a quart of water, you won't have the same color as one pound of C60.
And the flavor differences in the malts is pretty significant in that example too. Diluting down the raisin/dried fruit flavors of C80 or C120, don't get you to the light caramel flavors of C40 of C20. It just gives you diluted raisin/dried fruit flavors.
C75 is so close to C80, your best just doing a straight up sub.
Oh and before anyone asks, I've done these experiments before too.