If you want to be precise about it, I'd chart or draw on graph paper. EQ is great for estimating, but often will over estimate, which quite honestly is fine by me because I'd rather have too much than not enough.
Depends on the block and pattern. Smaller pieces will require more seams, which will eat up yardage. On the other hand, if you have a block with just one seam, that is very easy to measure and do the math on.