Sometimes your pattern tells you to decrease evenly and having to put your needles down and start calculating is not a thing most of us are eager to do. I have saved you some trouble and made this simple calculator. Just enter the number of stitches you have currently on your work and the number of stitches you want to decrease and the calculator will do the math for you.
sts on my needles and sts.
Decrease evenly across a round: (k12, k2tog) repeat 4 times.
Decrease evenly across a row (balanced decrease): k6, (k2tog, k12) repeat 3 times, k2tog, k6.
k = Knit
k2tog = Knit 2 stitches together