You could make 1/2x where x = 4 evaluate to either 2 or 0.125, but either choice will have surprising results in some cases or to some people. Perhaps a better choice, especially for an interactive calculator, is to adopt partially ordered operator precedence, and reject the expression as ambiguous. Then the user can insert parentheses to disambiguate.

In a CFG this is easy enough to express. Here's an untested and at any rate incomplete CFG demonstrating the technique:

    expr ::= term | term "+" expr | term "-" expr
    term ::= quotient | product
    quotient ::= atom "/" atom | atom "/" quotient
    product ::= atom product | atom "*" product | atom
    atom ::= number | variable | "-" atom | "(" expr ")"

But typical parser generators don't give you an easy way to provide a useful error message there.

That version of the grammar, btw, avoids left recursion, so it ought to work as a PEG, but at the cost of the wrong associativity for "/" and "-". You either need to use left recursion or restructure the syntax tree afterwards.