Text size for multiple devices?

When entering placeholder text in native textfields I was using a combo of the user’s device display.contentHeight and display.contentWidth to calculate text size. It works fine for some devices but disastrous for others. Is there a way I can have it proportional to the height of my textField box? Usually what happens now is that the placeholder text (and text when user enters it) is either too big or too small on certain devices. 

Or is there an even simpler way to do this? I’m all ears!

Thanks very much in advance