Being fairly new to used starters, I'm somewhat confused as to how much volume of water to boil when adding DME to create a starter. For instance, if the Kalkulator I'm using says to start with 2 L of water and add 8 ounces of DME and boil for 10 to 15 minutes, I will only have approximately 1.5 L of water at the end of that process. Does that mean I need to add back in another half liter of sterilized water to get to my 2000 mL volume from the calculator?
Yes, starter calculators are a post boil volume.
Your boil off depends on what you're boiling in (surface area mainly) and how hot you boil it.
Do a test to see how much your vessel and burner boils off in 15 min, then add that volume to your total starter volume for you wort boil.
I boil right in the flask and only get about 100-200ml boil off.
I don't know your calculator. No one here knows unless you will link to it. So we cannot tell what it's developers had in mind.
That said, you want about 1.040 OG / 10 Blg after boil. If that means starting with 2 liters and boiling it to 1.5, do it. If this means starting with 3l and boiling down to 2? So be it.
The one you linked gives you post-boil volume (verified using Brewtarget). But that's not the point. The point is to know what you are doing and what you need - and that's certain volume and certain gravity when you add your yeast.