Hey, I'm looking at buying a new guitar (specifically the Ibanez RG721) because I wanted to have one guitar that is tuned in standard/drop D and another that can be tuned between drop C# and drop B, but it appears that 10 - 42 don't quite make it to drop B (or at least they don't on the GRG 270 I have) and I was wondering what gauge would benefit me best for being tuned no lower than drop B
Maybe try a set of 11 gauge strings.
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justinguitar.com is the answer
It's all dependent on taste of course but for a low B on my 7 string I use a 60 or 62 if I can get it. That's probably a bit much for going to C# but I'd probably go with something in the 56 range for that.

That's me though, I like my lower strings beefy.
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I have to say I like very heavy strings, but I use a 11-70 set for drop B on one of my guitars. I do drop the B down to G# and F# for certain stuff.
I have used this set for Drop B: http://www.daddario.com/DADProductDetail.Page?ActiveID=3769&productid=26

Kinda loose, but it worked well enough. It may work for both tunings you are asking for.

If I was doing only Drop B, I would get: http://www.daddario.com/DADProductDetail.Page?ActiveID=3769&productid=31

Then I would swap the third for a wound string.
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