I'm not clever at all with Javascript so unfortunately the little fix I made to it (described below) took me all day to figure out. (I should've paid more attention in math class.)

I couldn't get my mind off the fact that filling in the New Height value while leaving the New Width blank produced a result that was incorrect. The answer, of course, came as one of those "doh!" moments once I finally got it.

Here are the 3 lines I finally zeroed in on:

var ratio = origwidth/origheight;
newheight = newwidth/ratio;
newwidth = newheight/ratio;

To get an accurate new width, you have to multiply it by the ratio:
newwidth = newheight*ratio;

Hope this helps. I also would appreciate it–since I'm weak both in math and Javascript–if someone points out any errors and/or omissions I might have made with this fix.

You can find a working example of my edited version of the Proportional Scaler script here: http://www.pixeljax.com/

I would like to see it as a result of the calculation and as an amount you could specify to get both the width and height. Used for figuring parts of a larger area, this would make this tool a fully functioning proportional scale.

Thanks for making it available.