Looks like a copy of the REM Calculator we made a few years ago. Wish it had occurred to me to make it into an extension!
I've been out of front-end for a while, and this question popped up in my head:
Is there a reason why you shouldn't set your body font size to 10px, always?
I recommend to not set the body font size, this setting is better to have it unset so the browser settings of the user will tell witch font-size to use.
How predictable is that?
It's not predictable. You know that will maintain the proportions always and get used to it. Usually the default browser font-size is 16px, so designing for this using rem and em's when it fit's best. This will ensure that if somebody have visual difficulties and need to have the font-size bigger will see your site correctly scaled proportionally.
Hey folks I made this Chrome extension some time ago for quickly converting px units to rem. Lemme know if you have any feedback for me.