I also have a Bosco like JohnB and use reduced steam flow for smaller amounts of dairy. I think this may be due to the big 6 liter boiler the Bosco has and the prodigious mount of dry steam it can produce. When I was using my VBM Domobar I generally had to steam wide open to get good results. Just my observations between the 2 machines.
Suffering from EAS (Espresso Acquisition Syndrome)