When harvesting, I just take a gallon sized ziplock type Baggie to line a half gallon plastic pitcher (to make it easier to pour into the Baggie). Then I eyeball the amount from there. I do a lot of ten gallon batches of same ABV range lagers, so for those, I will re-pitch at the rate of about half of the original harvested amount; ales go at around a third or less. If the harvested yeast has sat for over a month, I typically will make a starter, but not always; sometimes I will just pitch a bit more than the fractions above. YMMV, of course, and bigger beers of any type require more yeast to complete the process without excessive stress.