Substituting crystal malts?

[MNHazmat]

Is there an equation for determining how to substitute crystal malts for each other?

Example -- I have a recipe that calls for Crystal 15. My LHBS doesn't carry 15 but does have 10 and 20. So.... do I use more 10? Some 10 with a dash of 20? Half the amount of 20 that was called for as 15?

[snowcapt]

Im pretty sure Palmer covereds it in "How to Brew"
I remember it mentioning substituting different grains for the "same" result.
Say your recipe calls for #1 C80. You can get there by using #1/2 C120 and a #1/2 of C40.
So, I would guess your ratio to be 2:1. 2 parts C10 to 1 part C20.
Dig?
I could be way of, but not that far, I don't think. Not sure. Maybe.

[Bugeater]

Blending the different colors like Snowcapt says is something I do quite often. A side benefit (for me at least) is you get a little more complex flavor from the blend.

Wayne

[Kbar]

I am skeptical of trying to mix grains of different colors (depth of roasting/toasting) to do anyting more than just acheiving color correcting. They impart different flavors so expect that difference to exist, at the least.

[anday6]

Any color of crystal averages out to the stated color. The 10 probably has some percentage of grains that are 20L and the 20 probably has some percentage of 10L grains. Plus, since they are both low color and very similar in almost every way, there's just not a whole lot of a difference there. There would be a lot more to worry about with using 120 and 20 to blend for 60 because those flavors are very different.
In this case, you can probably blend it at half and half or whatever ratio you like.

[Afterlab]

Two things I might factor in would be the percentage of crystal malts you are using in relation to your base malt, as well as your ratio of crystal malts to other specialty grains.

If you brewed a simple Pale Ale with 5% c15 and 95% base malt you may not notice a huge flavor difference if you blend them like the others have mentioned. If you were doing an Amber Ale with 10% c15 you may want to play it safe and use the 2:1 ratio Snowcapt mentioned and favor your lower lovibond c10 malt.

If you are using c15 as 3% of your grain bill on top of a variety of other speciality grains, you probably won't notice a huge difference in the flavor if you substitute with c10 vs c20.