Yeast starter on stir plate, when to add?

I have a 5 gallon pumpkin ale batch that I brewed yesterday. I forgot to make the starter a couple days in advance and made it yesterday morning. I have it going on a stir plate. I boiled and cooled 1/2 cup of DME mixed with the yeast. Should I boil any more water and DME, cool it, and add that to the starter or should I just pitch what I have so far?
Should I turn the stir plate off and let it settle before pitching or just pitch directly from stir plate all mixed up?

At this point I would pitch the whole starter now. Your beer has been setting for a day and the starter, even if not finished, can only help.

It's OK to pitch directly from a single vial. You'll get where you want to be eventually, it just won't be ideal. If your starter isn't QUITE at 100% of it's potential, that's OK, and it will still be better than a single vial pitch. (Which, as a lazy SOB who can only get his act together to make a starter 3/5 times, isn't often a noticeable problem.)

How would I "know" when the starter is ready anyways?

On a stir plate the whirlpool will go away or be significantly reduced. You may notice a change in color towards white. Some krausen may form. Or maybe not.