I to was using 100gr DME per litre until I checked the gravity one day. At the end of 15min boil I had around 800ml of starter wort at 1.056 SG. I had never taken the boil into account. I now add in the 15min boil off volume (200ml) to my staters. I use 100 grams of DME for a 1200ml starter @ 15min and end up with 1000ml vol at ~1.037 SG.