pewc_number_field_step_decimals.php

<?php
/**
 * Filter the step parameter in Number fields to return decimals
 */
function prefix_number_field_step( $step, $item ) {
	return 0.01;
}
add_filter( 'pewc_number_field_step', 'prefix_number_field_step', 10, 2 );

/**
 * Filter the step parameter in some Number fields to return decimals
 */
function prefix_specific_number_field_step( $step, $item ) {
	if( $item['field_id'] == 1234 ) { // Change this to the ID of your field
		return 0.01;
	}
	return $step;
}
add_filter( 'pewc_number_field_step', 'prefix_specific_number_field_step', 10, 2 );

And for more flexibility (applying a custom step for some fields only), it's also possible to use either the 'field_id' or 'group_id' key of the $item array (here with field_id):

function prefix_number_field_step( $step, $item ) {
	switch ( $item['field_id'] ) {
		case 42:
		case 135:
			$step = 10;
			break;
		case 69:
			$step = 420;
			break;
	}
	
	return $step;
}