Are you saying that no validation occurs (no message stating "Value must be less than or equal to 25") after your javascript runs and inputs a number larger than 25 into the field?

Does the validation occur if you try to submit the form or if you place the focus into the field that stores the final value and then click or tab out of it?

The number validation won't run automatically, but if that's what you're looking for, you might want to consider doing the validation in javascript. Below is an example where it adds the values from a couple of fields and then stores the total into another field. If the total falls outside of the range of 0 to 25, then it will display a message as well as disable the "Submit" button. The fields I'm adding use the CSS class, sum, and the field that stores the total uses the CSS class, total.

$(document).ready(function () {
    $(document).on('blur change', '.sum input', sumtotal);
  
    function sumtotal() {
        var s = 0;
        $('.sum input').each(function () {
            s += parseNumber($(this).val());
        });
        $('.total input').val(s);
        if ((s < 0) || (s > 25)) {
          if ($('.totalWarning').length == 0) {
            $('<p class="totalWarning">The total must be between 0 and 25.</p>').insertAfter('.total');
            $('.Submit').attr('disabled', true);
          }
        }
        else {
          $('p.totalWarning').remove();
          $('.Submit').removeAttr('disabled');
        }
    }
    function parseNumber(n) {
        var f = parseFloat(n); //Convert to float number.
        return isNaN(f) ? 0 : f; //treat invalid input as 0;
    }
});