Is there a way to figure out what the approximate gauge is for a yarn that is doubled?
for instance, let’s say there’s a yarn that has a gauge of 4 sts = 1" on #7 needles.
Can I figure out what the gauge would be doubled? Would it just be a stitch count of half the original gauge (2 sts in my example)?
I don’t think it’s as simple as that, though this is really just off the top of my head. Since the two strands would most likely take up less space on the needle as the one strand due to the fact that the two strands are coming out of one stitch, I don’t think it would be exactly half. You could do a very quick swatch to just get an idea–at least that’s what I would do.
I think this is what you’re looking for:
http://www.thedietdiary.com/blog/lucia/189
If you don’t want to strain your noggin as I did, scroll down to the “summary” section and skip the explanation. 
Wow :shock:
That’s cool!
I’m glad someone has the brains to figure this stuff out, I sure don’t! 