Custom Filter Based On Other Field Value

[blueprint]

How can I implement a custom filter which is based around the value of another field?

For example, I have a field called `stockLevel` and another called `minStock`. I want to develop a filter whereby I can show all entity records whose `stockLevel` is less than `minStock`.

So far, I have this:

protected function filterBelowMinimumStock(&$result)
{
    $result['whereClause'][] = array(
    'stockLevel<' => <FIELD NAME HERE>
    );
}

However, I don't know what to do with the <FIELD NAME HERE> bit