Yeast starter volume after boil?

Made my first yeast starter today with the NB yeast starter kit 1000ml. The instructions say to boil 650 ml and the DME and boil for 15 minutes cool and pitch the yeast. When I added the mini wort to the flask I had lost almost half of the volume of liquid down to 400ml from the 650ml. Should I have added more water to get to the original volume of 650ml?

You could have added more water. Adding more water will keep the SG in tha area of 1.036. Your SG may be at around 1.050. The yeast will probably not be to stressed to multiply because of the higher SG, but your cell count will not increase as much with the lower volume of the starter.
A simper way to make a starter is to add the DME to boiled water, bring back to a boil, and remove from the heat. Water may contain some contaminants from the faucet. The boil will kill these contaminants. DME being almost pure sugar will not support bacterial growth. Sugar is a preservative. Any bacteria that could have been introduced, when adding the DME, would be killed by water that is over 160°F.

Weighing the DME is more accurate than using volume measures. ... alculator/

A 2000 ml flask is much safer to boil in. Their shape though is designed to create volcanic boil overs. It would be safer to just use the flask on the stir plate, and a kettle to dissolve the DME into boiling water.

Perfect that method sounds much easier and clearer than the included instructions. Thanks for the help!