It can be difficult to determine how much yeast you have. I usually crop the yeast into a quart mason jar(sanitized) and then add boiled(and then cooled) water to fill up the jar. I shake it up good and then loosen the lid a little to allow some gas to escape. I put this in the fridge for a day or so to allow the yeast to settle. I usually have a nice compact cake of yeast on the bottom of the jar. From there you can estimate how much yeast you have. And the viability will not be effected too much if you use it in a week or so.