How to use a Stylus variable in calc?

To use multiple variables (not just one) in calc (or with other functions), i use sprintf as you used, but with tuples:

arrow-size = 5px
measure = 50%
left "calc(%s - %s)" % (measure arrow-size)

Remember that interpolation in Stylus is supported through {} and it's used for other kind of interpolation. It's used to surround an expression, which then becomes part of a identifier, or a selector.

In order to use a Stylus variable inside a calc expression, one must employ the string % operator:

arrow-size = 5px
left "calc(50% - %s)" % arrow-size