+// generate (return) a "calc(Fpx + R%)" to scale linearly with parent
+// args:
+// pb: pixel width of parent (when biggest)
+// ps: pixel width of parent (when smallest)
+// cb: pixel width of child (when biggest)
+// cs: pixel width of child (when smallest)
+linear_scale_calc(pb, ps, cb, cs)
+ // math explained: (figuring out F and R in calc(Fpx + R%))
+ // f + r * pb = cb // known_1: formula(pb) -> cb
+ // f = cb - r * pb // solve_for_f: subtract (r * pb) from both sides
+ // f + r * ps = cs // known_2: formula(pb) -> cb
+ // cb - r * pb + r * ps = cs // substitute solved_for_f into known_2
+ // -r * pb + r * ps = cs - cb // subtract cb from both sides
+ // r * (-pb + ps) = cs - cb // factor out r from left side
+ // r = (cs - cb) / (-pb + ps) // divide both sides by (-pb + ps)
+ // r = (cb - cs) / (pb - ps) // multiply left side by -1/-1
+ r = (cb - cs) / (pb - ps)
+ f = cb - r * pb
+ unquote("calc(" + unit(f, "px") + ' + ' + unit(100 * r, "%") + ")")
+