If both Product Add-Ons Ultimate and Fees and Discounts are installed, and "Apply User Role Pricing to add-on field prices" is enabled, you can override this on a per-field basis using the "wcfad_pewc_adjust_field_prices" filter

wcfad_pewc_adjust_field_prices.php

<?php
add_filter( 'wcfad_pewc_adjust_field_prices', 'custom_wcfad_pewc_adjust_field_prices', 10, 3 );
function custom_wcfad_pewc_adjust_field_prices( $adjust, $item, $product_id ) {
  // if field price is a percentage of the product price, do not adjust
	if ( $item['field_percentage'] ) {
		$adjust = false;
	}
	return $adjust;
}