How do you determine a dividend payout ratio?

That term could mean a couple or three different things, depending on the context. But most commonly it refers to the ratio \frac{p}{n} where

p = total dividends paid out during the period
n = total net profit for that same period

In other words, it's the percentage of the earnings that were paid out to the common shareholders. Note that

(1 - payout ratio) = "plowback rate"; the percentage of the earnings that were reinvested in the company's assets and projects. I mention it because if you're dealing with payout ratio, you'll encounter plowback rate before you turn the page.

Hope that helps a bit,