A simpler trick than previous ones: calc(0px - something) – with an unit – works while calc(0 - something) doesn’t. See Fiddle 3
These “tricks” work:
calc(1px - something - 1px);
calc(-1 * something)
calc(0px - something) /* new */
where 0 - something didn’t (at least with your example).
Fiddle 1
Fiddle 2