well you can using an old excel4 macro but you need it to be used in a named formula
where exactly is this data you want to convert? it matters because of the construction of the named formula
but lets assume its col b define a name as say"mysum"
=EVALUATE(SUBSTITUTE(Sheet1!$b1,"X","*"))
then with 1-3/16X12 in b2
=mysum in c2 will give
-1.25