I am not great at gauging when I knit, in general, I have been lucky so far and I haven’t had any problems with my projects. I have knit a cardigan before by a pattern and it came out well, and now i want to design a cardigan of my own, i know how i want to look, but my problem is figuring out how to gauge the pattern so it will be the right size. I made 2 swatches, one in a rib stitch and the other stockinette. I measured an inch from the swatch and found that in ST st I need 8 st for every inch and in the rib stitch i need 9. Then I multiplied the length by the st #, and the width by the st #. Is that right, or is this just ridiculous.
Right, figure our your sts/inch and multiply times the measurement you need. If the body is in stockinette, use that to figure out your sts, just knit the ribbing on a needle one size smaller. The length will go by measurement not rows.
Thank you sooo much, I’m so excited to try something on my own now, I think I’m finally starting to catch on. 
Chances are, you may be more accurate by measuring over 4"X4" to get the measurement you want. After that–What suzeeq said—.