How to Calculate Cast On Stitches

So, the body of my cardigan design is complete and now it's onto the sleeves! It's a fairly mindless knit-in-the-round kind of deal, but figuring out how many stitches to cast on required a bit of math.

I've been using Elizabeth Zimmerman's Seamless Raglan instructions to work out the precise measurements and how many stitches to cast on for each part of the pattern. If you've read Knitting Without Tears, you know EZ works with percentages. I am by NO means a mathematician, but there's one thing I've taken away from math class, that I keep coming back to, especially when knitting is this formula:
[Part = Whole x Percent]

For example. Let's say you've cast on 200 stitches for the body of a sweater. EZ says cast on 20% of the body's stitches for one sleeve. Here's how you break it down:

Part = Sleeve Stitches
Whole = 200 body stitches
Percent = 20% (There's an invisible decimal to the right of 20. Move this decimal two digits to the left. 20% now becomes .2)

So this is what your formula should now look like:
Part = 200 x .2
200 x .2 = 40
So, 40 is the number of stitches you need to cast on for each sleeve. Easy-peasy, huh?

You may have already known this, or my apologies if I just made your brain hurt but as someone who is totally not a math person, I figure I'd share the very little math I do know... if it's going to help you get your knit on. ;-)