Sorry this equation does work - the reason that Actinic is not measured in Kelvins is that part of the spectrum is filtered by using Actinic reacting gases in the flourescent tube and therefore the spectrum is limited to a narrow bandwidth i.e. does not exactly follow the "black-object-heated" rule of spectrum vs. temperature.
However, the rating 420nm exactly means that that is the PEAK WAVELENGTH of the light emitted by the bulb (however at a narrow bandwidth). So the temperature of the light is meaningful but those unfamiliar with its meaning and application think that it does not apply.
"goes up to 20,000k" You mention that 420nm is far beyond that - what does that mean??? are you talking about visible spectrum or radiation in general. The statement about it being far beyond that is totally meaningless. The visible spectrum is about 320nm to 720nm (about 10,000k to 4,000k).
Also be aware that it works the other way, for example:
2900000/10,000k = 290nm Peak wave length for a 10,000k Bulb.
household bulbs of approx. 5000K:
2,900,000/ 5000k = 580nm
This is a very well known equation for converting temperature to wavelength and visa versa - you will find it in any physics 101 class. It does apply to actinic bulbs as well as long as you understand the meaning of temperature and light and that Actinic has a peak output but has a narrow bandwidth as well.
But thanks for your interest.