When saving yeast from a previous batch how much will I want to pitch? Like if I collect a cup or two in a mason jar and put it in the fridge will I want to pitch all of it or will that be over pitching ...its tough to know how many billion cells are on must yeast cake if that makes sense..
As a rule of thumb, if using slurry within a month of collecting it, I use about a quarter of the old cake. I don't make a starter; just pitch it. After a month, I use the same amount, but make a starter to ensure the yeast is viable.
Some pint jars have milliliter measurements on the side. Mr. Malty will tell you, for example, you need 50 ml of thick slurry. Once the yeast in your jar packs down to a thick slurry on the bottom, you can compare the volume of that slurry to those measurements. It's not laboratory-precise but I've never had a problem doing it that way.
Who is this Rorschach guy? And why did he paint so many pictures of my parents fighting?
I have always just poured the whole jar into the fermenter. I have also poured fresh wort into a newly emptied fermenter right on the old cake. I am sure someone will tell me why it's not true but I just don't believe in overpitching for 5 or 10 gallon batches of beer using slurry from 5 or 10 gallons of beer.
What else are you looking for? I think Jon's method of using the Mr. Malty calc in conjunction with a volume measurement is probably the most consistent. You can end up with more slurry in one jar than another so it's good to have some sort of measurement.