The other biggest advantage to the “62.5%” trick is that not only does it make using rems in units of 10 easier, BUT it ensures that if your user is visually impaired and they have their base browser font size set to something besides the usual 16px that your font sizes scale to what they prefer. This was used much more when browsers couldn’t scale font sizes set in px. Still seems reasonable to me, but with calc() and var() there are a lot of possibilities.