Is 300 one of those 'magical' all encompassing numbers (such as 127)?
Nope. That 300 is the value of the width (in pixels - dots of light on your monitor screen) of the text you want to glow.
All the magic numbers are multiples of 8 ( or 16, which is itself a multiple of 8 ) or their computer representations. Remember in computers everything starts at 0 instead of 1, so 127 is often considered #128 because there are 128 numbers in the sequence 0, 1, 2, ... 127.