The most important aspect of repitching is having the yeast replicate 3-5 times,this gives the most viable cells. I use Brewdads calculator because you can change all the inputs and it has a growth factor slot.
1- input your OG and quantity the normal way and look at the cell count needed
2- make the starter (on the computer) note the amount of cells.
3- now step that starter up to your final batch size off stirplate.
4- now look at the number of cells and the growth factor,it should be 3-5 times
5- all you have to do to find out how much of the yeast cake to repitch is divide the final cell count by the cell count you needed to pitch into the first batch
I haven't done it for awhile,when I first found this and played around with the numbers I found my SOP and that is ales 1/4 , German ales 1/3, lagers 1/2 of the cake on a 1.050 wort.
