Tube Testing ? for 300B Type - help needed

Good day tube lovers!

On my Hickok tester on the roll chart is the knob settings for testing tubes. I am having difficulty translating the 300B tube for a correct and positive reading.

The last column on the roll chart for a 300B tube under "Notations" states "@ 25%" My question is...25% of what??

If anyone can shead some light or offer an explanation for this "notation," I would be very grateful.