How to calculate minimum image scale?

I need to setup a jQuery UI slider as 开发者_开发知识库an image scaler, yet I need to make sure that it is restricted on the low end to meet minimum size requirements.

Given that I know the following:

• existing image dimensions
• minimum width and height
• all scales are to be proportional

... how do I calculate the minimum scale (as in a percentage) so that scale of the image meets the minimum width and height requirements?

Let me know if that wasn't clear, and I'll try to clarify.

var minWidth = 100,
   minHeight = 100,
    curWidth = 600,
   curHeight = 800,
        minW = minWidth / curWidth,
        minH = minHeight / curHeight 

    minScale = Math.max(minW, minH); // .1666666666