Determining Above the Fold in different browsers, themes and designs?

Lately I have been thinking about the whole "Above the Fold" thing, and the fact that there is an algo for the amount of ads above the fold in relation to the actual content.

What is considered above the fold?

I mean, we know what it means, but with all the different themes and designs, as well as how different browser display the same designs differently, how do you know if you are staying in line. You know what I mean?...

Now, of course you use common sense and say well if all you can see above the fold is ads, then it's not good.

But what if one browser displays your design in a way that a few paragraph of your content are above the fold, while another browser display it in a way that only the ad is above the fold?

I am not really taking it that seriously, and I am sure small differences like that won't have any major impact in how you rank or whether or not you are penalized, but it has been on my mind and I would like to see what other people think about this.

Now, I know nobody can really has the answer beside Google, but I like to get your opinion on this.

Thanks in advance.