After playing about with RSI's own examples, I now have a widget whereby you can set slider to two positions by using corresponding buttons. It's about as complex as JavaScript can be.

Some way off from having something which will behave as I want it to (i.e. you put in certain known/fixed/ideal values, and vary/covary the others to help you understand the limitations of your instrument design), but every journey starts with a single step.

I make it nearly thirteen hours today.

Eurgh....