Prorate the initial payment. Useful for subscriptions that occur on the first of every month.

pmpro-prorate-initial-payment.php

function my_pmpro_checkout_level($level)
{	
	$current_day = date('j');
	$days_in_month = date('t');
	
	$level->initial_payment = $level->initial_payment*(($days_in_month - $current_day)/$days_in_month);
	
	return $level;
}

add_filter('pmpro_checkout_level', 'my_pmpro_checkout_level');