thanks for your answers! I diluted the starter yesterday evening and later stepped to 1 liter (1.040 gravity) today in the morning. I put it on the stir plate. I plan to brew on Sunday, so I hope the yeast will grow by then. I also plan to freeze some of the yeast, so I have to put the yeast in the fridge in about 36 hours from now, so they'll drop out of suspension.
@shertz: I used 17g of light DME for 100ml of wort, and then added about 100 ml of clean mineral water.