I am a beginning weaver. Not only that, but I am not good at math. Is there a VERY simple way to calculate the amount of yarn that I will need for a weft?
Lots of us are not all that good at math. Luckily, this is not too hard to figure out. To determine the amount of weft for a project, first you need to know the picks (weft threads) per inch. If it's plain weave, twill, or lace weaves, this is usually the same number as warp threads per inch. Then you multiply that number (picks per inch) times the number of inches you'll be weaving the piece times the width of the weaving plus about 10% (for example, by 11 inches for something 10 inches wide). Then you divide this number by 36 inches to get the number of total yards needed. A calculator makes this easy.
If the tangling is happening when you are warping, it isn't the yarn, but the methods you are using. I think the video Warping Your Loom might help. 8/2 cotton should be pretty easy to use with good warping methods.
And welcome to weaving!