[BTS] Calculating Whip Overflow in slavery

As a rule of thumb, I know that you generally don't what to whip or rush on the first turn you started building something (unless it's a city that you just took over and those people would starve anyway.

I also know that big whips are better than small whips (better to 5 whip that 10 pop city than 1 whip it.


But I've been watching quite a few of chris67132's videos:

http://www.youtube.com/user/Chris67132/videos

and in them he is often making a calculation about maximizing whip overflow hammers.

For example, right here:

http://www.youtube.com/watch?v=O1OZAuE6DkA&t=45s

I'm not clear on how to make that calculation. Can someone help me?