On the Elizabeth, the purge was 4g of water.
There was a 21g difference in weight between the weight of the milk jug before and after steaming. So, in total, steaming 250g of milk drew 21ml from the boiler (or 25ml if counting the initial purge).
Edit: I tried again, this time purging the wand properly rather than trying to keep the steam inside an cup! I the difference was 12g, more in line with what I remember recording in the past. I could only assume the other 9g of water would therefore be part of the initial purge.
