Here is how I have been doing it (full disclosure: I am a bit anal about this). I weigh out 100 grams of DME for each liter of water to make a starter that is 10 degrees Plato (1.040) which I have gleaned from literature and manufacturer's recommendations. Let's assume a 1 liter starter (1.06 quarts). I fill the flask with maybe 700 ml of water (since the DME addition will increase the volume in the flask), heat it to slightly below boiling, add the DME, and swirl to dissolve it. I then top up the flask to 1 liter with additional water. That gives me the 10 degree Plato number I am looking for (within a small margin of error).
If that sounds like way too much work, you can keep doing what you are doing. Your volume will be a bit more than 1 liter and your starter gravity will be slightly lower, but you will be close enough.