add_action( 'woocommerce_cart_calculate_fees', 'custom_cart_total' );
function custom_cart_total($order_total) {
  $user = wp_get_current_user();

  if( in_array( 'practitioner', (array) $user->roles) ):
    $tradeDiscountTotal = 0;
    $loggedInUser = get_current_user_id();

    //Get discount from user & work out numerical value as a percentage
    $discountApplied = get_field('trade_discount', 'user_'. $loggedInUser );
    $discountApplied = 100 / $discountApplied;

    foreach( WC()->cart->get_cart() as $cart_item ) {
      if(!get_field('trade_discount_exempt', $cart_item['product_id'])) {
        $tradeDiscountTotal += $cart_item['line_total'];
      }
    }

    //Work out discount using user discount & update the price
    $totalDiscount = $tradeDiscountTotal / $discountApplied;
    WC()->cart->add_fee( __('Trade Discount', 'woocommerce'), -$totalDiscount );
  endif;
}