Math is a Four Letter Word

It really is. I hate math, and it looks like, with the help of a couple of my knitting books, I'm going to have to do some calculations in order to get a sweater that fits with a needle size too small. My cactus sweater calls for size 8 circs, and a gauge of 16 sts to 24 rows for a 4x4 swatch. I got 20 sts to 24. Need I say more. I don't have a size 9 circ, nor do I ever recall having seen on in the stores. PLus, (with an extra PL) I like the way the fabric knits up at 20 sts. I'm afraid if I do find a needle and go up a size, I won't like the resulting fabric.

My hypothesized solution is to calculate how many sts I will have for a 40" sweater, as opposed to what is called for, and compare it to the next size smaller. If I'm getting an extra stitch per inch of fabric, it may be possible to still use the size 8's following the directions for a size smaller, and still get a sweater that fits. Please tell me that makes sense. I wish I had someone grounded in the real world to propose my theory to, and whom could straighten me out a bit. I have a feeling I"m going to have to learn by trial and error, as usual.

Actually, now that I think about it, if I have more sts, shouldn't I follow directions for a bigger size, with the result of the size I actually need???
Oh God....
IS ANYONE OUT THERE GOOD AT FIGURING THIS STUFF OUT??????