Trying to Scale font size down as words are added

I have a text layer in a text box. With fewer words I want the text to be at a maximum font size and as more words (and lines) are added I want the font size to scale down to a minumum size.

I have a null controller with a slider control (output) with this code

var driver = thisComp.layer("Display Text").sourceRectAtTime().height;

var dMin = 845;

var dMax = 1300;

var pMin = 305;

var pMax = 240;

linear(driver, dMin, dMax, pMin, pMax)

On the source text of text layer I have this code.

var FS = thisComp.layer("Controller").effect("Output")("Slider"); text.sourceText.style.setFontSize(FS).setLeading(FS)

When I add words and lines I can see in the properties panel that the font size is going down, but it still displays at the original (max) font size.