Good to hear you are firing up some Ikes. I might start at 50 or 55mV, and then take a plate voltage reading. Let's say you adjust it to 55mv and then read 480V at the plate. That means we are dissipating 480v * .055A or 26.4W on your
6550, which has a max dissipation of around 48W. I might shoot for 70% of max dissipation which is 33.6W as a goal. So then I'd adjust in 5mA (ie 5mV) increments upwards until I got close to 33.6W at idle. You have to carefully measure your plate voltage, too, to see how lossy your power transformer is. But you only need to do this during the first tune up.