Best practice for scaling font size

[Jon Heron]

Hello,
What is the best practice to scale text on top of an image?
I am building a calculator and I would like the text to scale if the number of digits displayed overfills the display, as well as for screen rotation from landscape to portrait. I have several text boxes overlayed over the screen image so I need the text to scale proportionality to the image.
Scaling the screen image is easy with height and width properties but fonts only have either PointSize or pixel Size...
Currently I am scaling the font as follows, it works fine until I enter portrait mode ands then the display text is truncated instead of scaling down.

    
Text {
        id: memText
        property double memtxt:.22;   //for scaling font size
        anchors { right: parent.right; rightMargin: parent.width * .02; verticalCenterOffset: parent.height * -.28; verticalCenter: parent.verticalCenter }
        onTextChanged:  {   //scale the input text
            var length = text.length;
            if (length < 5){
                memtxt = .22
            } else if (length == 6){
                memtxt = .21
            }else if(length == 7){
                memtxt = .18
            }else if(length == 8){
                memtxt = .16
            }else if(length == 9){
               memtxt = .14
            }else if(length == 10){
                memtxt = .12
            }else if(length >= 11){
                memtxt = .10
            }
        }
        font.pixelSize: parent.width > parent.height ? parent.height * memtxt : parent.width * memtxt;

This works fine if I lock the app to landscape view, However, its far from smooth and I feel as though I am reinventing the wheel here, there must be a better way to do this smoothly and efficiently...
Thanks for any input!

Cheers,
Jon
Here is an image of the screen image and text box's to give an idea of what I am up to....
Screenshot-2.png