Right. If you want a solution that's got 2.5 milligrams for every 10 milliliters, but you must measure 10 times as much mass, you must increase the volume by 10 to keep the ratio the same.
Then, taking one tenth of that should get you back to 2.5 milligrams per 10 ml.
You can then add as much water as you want, but the mass of solute won't change. If you want to end up with 2.5 milligrams per liter, just make sure you have 2.5 milligrams, and then dilute to the needed volume.