I like to "do my own thing" with flour, water, salt, and sometimes yeast.  This means that I often need to figure out hydration levels of a dough with 1 or 2 pre-ferments.  I could do it, usually, but it pushed the bounds of my arithmatic skills.  My husband wrote a javascript hydration calculator for me.  I use it all the time.  Perhaps someone else will find it useful as well. 

http://members.shaw.ca/breadsimple

:-Paul