Set the product weight based on the value of a field

prefix_save_product_weight.php

<?php
/**
 * Set the product weight based on the value of a field
 */
function prefix_save_product_weight( $cart_item_data, $item, $group_id, $field_id, $value ) {

	// Change the ID here to your calculation field ID
	if( $field_id == 7489 ) {
		$cart_item_data['product_extras']['weight'] = $value;
	}

	return $cart_item_data;

}
add_filter( 'pewc_filter_end_add_cart_item_data', 'prefix_save_product_weight', 10, 5 );

// Update the product weight
function prefix_set_product_weight( $cart ) {

	if ( is_admin() && ! defined( 'DOING_AJAX' ) ) {
		return;
	}

	if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 ) {
		return;
	}

	foreach( $cart->get_cart() as $cart_item ) {
		if ( isset($cart_item['product_extras']['weight']) ) {
			$cart_item['data']->set_weight($cart_item['product_extras']['weight']);
		}
	}

}
add_filter( 'woocommerce_before_calculate_totals', 'prefix_set_product_weight', 100 );