zoom % conversion to font size

I was reading an article about how to avoid eye strains when using a computer. One of the suggestion they gave was that the screen font size should be three times the smallest size you can read from your normal viewing position.

Internet Explorer offers a zooming feature when viewing webpages. The options they give are in percentages. How do you figure out the corresponding conversion of the percentages to font size?

screenshot of IE zooming options of IE9, the version I'm currently using:

ps i'm not sure which tag to put, so i put arithmetic.

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Please don't spend too much time on this. Just adjust your text bigger if your eyes feel strained!!! –  rschwieb Nov 8 '12 at 17:38
I don't know, sometimes it feels too big and sometimes it feels too small. I've adjusted the brightness to correspond to the ambient lighting, but just still doesn't feel right sometimes. –  Theo Nov 8 '12 at 17:53
You need one more piece of information, for example the font size you are starting from. Often the web page or some default in your browser (or other program) will set it. If you want $12$ point type and the page starts with $9$ point, you want a zoom of $\frac {12}9\approx 133\%$