As described on reddit.com by user Wadsworth. The theory is that for every video on youtube you can skip the first 30% without having a negative effect of the content. This has since been extrapolated to written content as well.
"There are 55 characters including spaces in this sentence.
According to the Wadsworth Constant we should remove the first 16.5 characters, let's give it a go.
ld be known as the Wadsworth Constant.
It is also known in the Wadsworth Constant that we should round to the nearest logical point. We are left with:
Be known as the Wadsworth Constant."
Source of quote: notsofatthrowaway