Hi, Nutty -- Once you fix the type and weight of the oils and choose a lye discount (also known as superfat; 5% is common) -- then that fixes the amount of lye you need. Once the fat and lye weights are set in stone, you can use several ways to calculate the amount of water in a recipe. You can have more water or less water in any given recipe depending on your preferences and on what you want to do with the soap.
Remember -- the kinds and weights of fats and the weight of lye are fixed and unchanging for any given recipe; only the water can be tweaked to suit your liking.
The usual way soapers determine the amount of water is to use a setting called "water as % of oils". The default in most
soap calculators is to use 38% water as % of oils. There are reasons why I don't recommend using this particular type of setting, so I'm going to stop here. I'll point you to a full explanation later.
Another (and IMO better) way of calculating the water is to use a setting called "lye concentration" or "water:lye ratio". Both of these mean the exact same thing; they just look a little different. My suggestion is to pick one and stick with it. I use lye concentration, so that's what I will talk about.
A common setting for lye concentration is the 30% you mentioned. I'd say a typical soap recipe for many soapers will have a lye concentration from 30% to 33%. Some use a little more water even -- a lye concentration of 27-28% perhaps -- and some use less water -- say a lye concentration of 40% or even higher. The most concentrated lye solution (least amount of water) you can choose is about 50% lye concentration. If you try to reduce the water to less that 50%, the lye won't dissolve properly.
So the shorter answer is -- a 30% lye concentration is one that is a pretty common one to use, but it's certainly not the only option. It would be a concentration I would be comfortable using if I wanted to do a swirl or other more complicated design using a reliable recipe. Other people might do the exact same recipe and design with a lye concentration ranging anywhere from 28% to 40%.
ETA -- Here's more info if you're interested in the details:
http://www.soapmakingforum.com/showthread.php?t=54095 Post 12
http://www.soapmakingforum.com/showthread.php?t=53642
http://www.soapmakingforum.com/showthread.php?p=530575
ETA2 -- And here's a recent thread with some background on the 0.40 Rule that Newbie talked about:
http://www.soapmakingforum.com/showthread.php?t=58581