Projection using externally defined value

I want to emit an event after so many of another event has occurred, where the count needs to be able to be changed through user configuration (e.g. it’s a batch size for an export process) , possibly also where the criteria for incrementing the count have to be derived from some external configuration (e.g. user sets a filter to ensure only the events with the right criteria increment the count) - can a projection be fed external parameters? Can I read another stream (e.g. get the last event from another stream that holds the parameters)

Ultimately i could use a subscriber to do this but if it could be done on a projection 'twould be marvellous.

Cheers in advance