How is starter size calculated?

I've seen the calculators and such, but I'm trying to figure out how they come up with the volume they do. I think I understand the first part of wanting to pitch proportionally to the density of sugars and the volume the yeast will be spread through and that the 0.75 constant for ales and 1.5 constant for lagers represents the ideal workload for each cell.

I don't understand how to go from the number of cells you want and the number you have in your packet, to the volume of starter needed. I'm guessing that since your starter gravity is optimized at 1.030 or so, you choose the volume that gives the yeast enough sugar to divide the appropriate number of times and then restock its reserves for dormancy. But how much sugar per cell per division does that take?

Yep the bigger the starter the higher the cell yield. I use a chart in one of my books "How To Brew". To basically give you an idea a 1.060 OG brew needs about 230 billion cells. Wyeast or White Labs liquids have about 80-100 billion when you buy them. My chart says that with 100 billion in a 1 qt starter = 168 billion. 100 billion in a 2 qt starter = 226 billion. and so on...... You could also begin with a 1 qt starter and then pitch that into a fresh 1 qt starter to increase count.

I started by using Mr Malty calculator on the internet but have since been using my book and a chart I made up. Once you get a rough idea you are good to go. I usually try to get close and in the past have heard bad things about over pitching yeast but have also read that you have to dramatically over pitch to get any bad results from it.