Made up a yeast starter of 560 billion cells on Thursday ready for brew day today. Had a disaster of a day and i am now not able to brew for about 2 weeks. Will this be OK until then in the fridge, it's in an Erlenmeyer flask with tin foil wrapped round the top? Also i guess the viabilty of the yeast will go down, how can i calculate how much of a starter i would need to get it back up to 560 billion