I'm in the same boat - washed WLP400 on 3/5 and put it in the fridge. It seems to be a guessing game with how many cells/ml and non-yeast % though.
I put an empty mason jar next to the jars with yeast in them and filled the jar with water to match the line of the yeast cake and then measured the water. I'm pitching about about 57 ml of slurry into a 1L starter that I boiled with 100 grams of DME. I'm going to put it on the stir plate for about 2 days, refrigerate for about 24 hours and then decant and pitch into a batch on Sunday. Any reason not to go about it this way?
I started a thread on this same subject earlier today and have been getting a number of different responses: