How would you add a variable defined in a customsite config to this?

For example if you set a exchange rate in a CustomSiteConfig, then wanted to use in this where the $num is?

in Page.php :

function eaPayment() {
$num = 4; //replace this value with a variable from customsite config UK below
$total = $this->Total;
$ea = $total/$num;
return $ea;
}

in CustomSiteConfig.php :

class CustomSiteConfig extends DataObjectDecorator {

function extraStatics() {
return array(
'db' => array(

class CustomSiteConfig extends DataObjectDecorator {

function extraStatics() {
return array(
'db' => array(
'Uk' => 'Decimal'
)
);
}

Thanks in advance!

Thanks, that worked!