This addresses an omission from Part1, showing how to adjust the zoom factor from 1.0 according to how far away you sit from your monitor. This gets slightly more mathematical than Part 1 but you don't need to understand it, you just need to plug your own numbers into the formula to calculate the zoom factor.
(To get the 'arctan' function on Windows 7 calculator you need to press the 'Inv' button and then the 'tan-1' button!)
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https://www.youtube.com/watch?v=ikwToOzX_UA