So 110100100 is broken as 110 100 100, or 644
as far as chmod (back on topic), in case anyone was lost by this, each of the bits represents the positions as pointed out in other posts: rwxrwxrwx - match up the characters positions - if you want it on (you want an r, w or x in that spot), put in a 1, if you want it off (the equivalent to a - in the permissions display), put in a 0. Then use all that new octal math you are now an expert in and you have your number. You can *add* them up, but that is not what is really going on, internally it is bit positions, not the result of a mathematical equation.
Jale.