Converting Widget - White Temp to Color Channels

OpenHab 3.2
Raspbian (will change eventually, but unlikely to affect this question)
Widget creation that modifies input to a custom output using , in this case a ZWave RGBW LED controller, could be done with any RGBW controller that is manually wired to the light strips though.

Item type therefore is Color
White LED Light strip has 2 channels (3 leads, Anode, +2 cathodes) one for White and one for Warm White.
Wiring is
White = Red Channel of controller 6500Kelvin
Warm White = Blue Channel 3500Kelvin

I need to write a widget slider object which will take the three properties props.coloritem props.white and props.warmwhite
which will be set as follows for the example case

Display value will be something like:

=((100-(items[props.coloritem].toRGB().[props.white]/2.55)) +(items[props.coloritem].toRGB().[props.warmwhite]/2.55))*(items[props.brightnessitem]/100)

and when the slider changes I need to take the slider value and transform it like:


I need help understanding the following given the above data:
Where/what tag do I place the slider value formula?
Where to I put the code for the reverse transform when the slider is changed in the UI?
Finally (and I may figure this one out quickly on my own in a minute) what var/item/prop/value do I enter to get the new slider value when someone changes it?

It sounds to me like this process is best achieved through proxy items and a rule. The two-way conversion that you are looking for doesn’t really exist withing the widget system. You can transform a value and display it differently in some text field, but if an item is linked to a widget then you don’t really have an opportunity to transform the value either inbound or outbound.

I was afraid of that. Thank you for confirming.