How to return value from a function launched inside a switch statement?

I have the following switch statement. All the cases work including the ‘custom’ case. the only challenge is that I am unable to return the radval from the ‘custom’ case. It shows the right value if I do it in the console log. I have tried several options mentioned in the comment below but none works.

    document.body.addEventListener('change', function (e) {
            let target = e.target;
            let responsible;
            let radval = 0.5;;
            let yourshare;
            switch (target.id) {
            case '100':
                    radval = 100/100;
                    break;
            case '50':
                    radval = 50/100;
                    break;
            case '25':
                    radval = 25/100;
                    break;
            case '75':
                    radval = 55/100;
                    break;

            case '33':
                    radval = 33/100;
                    break;
            case '0':
                    radval = 0;
                    break;
            case 'custom': 
                    document.getElementById('customsplit').onchange = function () {
                            let custval = document.getElementById("customsplit");
                            radval = custval.value / 100;
                            return(radval);  
// Also tried return radval and tried a function outside the switch statement that was called here.  But no success.
                    }
                   break;

    }