I have been all-grain brewing for about six years. I usually did 10-gallon batches but started deliberately making 11-gallon batches and canning the extra gallon of wort in one-quart mason jars. I use the unfermented wort to make liquid yeast starters. This way I can honestly say that my entire brewing process is all-grain.
Because the object of making starters is to grow more yeast, how big an increase in starter volume is required to grow more yeast? It seems that too small an increase in starter volume will only feed the yeast cells already there with no increase in cell population. The cells that are there will just eat the new wort and that will be that.
I usually use a factor of 10. If the initial package contains 50 milliliters of starter, I pitch it to a 500 milliliter starter. If I step it up again it will be to 5,000 milliliters of starter. Is this the best way?