This is probably the number one thing I get to do every week. And it doesn't get old.
"How do I find the best set of strings for (insert tuning here)?"
This is one of those things that's very specific on the customer in question, and what they prefer.
For ease, I've compiled all of our string tension info into a couple different charts, available at the top of each guitar string section (they're also listed below).
What you want to do first is find the set of GHS Strings that you use in standard tuning (let's say, GBL Boomers). Find the tension of those on the chart, and write that down; you'll be referring to that. From there, use the single string chart and find the tuning you want AT the tension you just wrote down.
The ideal situation is to create a set of strings for (example) Drop C that has the same tension as your set of GBLs. This will allow you to switch between guitars a lot easier than if you just threw a different set on.
That's the Readers Digest version. Please feel free to contact me if you have any other questions, or comments regarding this. I'm glad to help out.
String Tension Charts