.... it's so much easier do work with in your head:
deg Plato = weight of extract x 100%/ (weight of extract + weight of water)
For lower gravities you can even aproximate it with weight of extract x100% / weight of water
I realized this when I was making a starter last night. I knew that I wanted to have the gravity of the starter around 9 deg Plato. Since I was using 2L of water I knew that I had to add a little less than 200g of extract.
Before, I even thought of printing a table that shows me how much DME to use with how much water to get a specific gravity.
I guess it has to do with the brewing literature available to us. Most of the english books and recipes I read use specific gravity and all of the german recipes and books I've seen so far use deg Plato. That's why I seem to have a stronger urge to use deg Plato anyway.
(I just had to use them