Sorry dude but that formula of yours isn't quite correct at all, I've got to make a poster and the results isn't very exactly. I'm not saying is not useful is just that it isn´t perfect :S , I'm just saying is just one part of the process...

I found another way to size your slide poster style.... first you got to go to Keynote-Preferences and click on the tab "rulers", on the first box where it says "ruler units", choose either centimeters or inches instead of pixels and un-check the other two boxes below, leave alone all the other options.

Once you do that go to the menu bar-View-Show Ruler; the ruler will show in the units you ask for in past step.. so there you are... you are going to have your ruler show on your slide so you can achieved the size you're looking for with exact measurements... obviously the only way to do this is by changing the width and height on pixels unit as told before, so the formula given before could make this important approach to exact size you´re looking for....