Use a seperate 1/2 cup of DME for the starter. Making it about twelve hours before you pitch it won't make a lot of difference, but it will at least get the yeast started a little earlier. What exactly was your headache? A really long lag time? I've had starters that were going strong in less than a day, and ones that took three days to get going, so it's hard to say.
The last time I used WLP300, it was a bad vial, and didn't work, even after two days in the starter and three days in the carboy. I had to get a replacement vial from the LHBS, but that fermented like a champ, and now I have a delicious beer. Don't let a long lag time concern you TOO much, as your beer will more than likely come out just fine anyway.