I didn't use the whole cake, I think I used about 600ml slurry for 45 L? I'll double check that.If I were to use the slurry, is it just a case of running the wort straight on to it? Also, if the slurry is unprotected (i.e. the beer it has fermented has been racked) how long will it last for in the FV?
I would take the slurry out of the FV and store it in a jar in the fridge, then just throw it into the next batch, should be alright for up to two weeks. Sooner you use it the better.
Tested my alkalinity this morning and it's down at about 18ppm so no acid treatment. It was literally about a drop and a half of the KH reagent in the Salifert kit before it turned pink.