PHP code

Hi there,

How do I let this function know that the fieldname21 is okay to be blank but if filled in it has to be above 2.5 and below 545 ?

(function(){
if(fieldname21 > 545 || fieldname21 < 2.5 || fieldname21 != “1”) return ‘Price on its way via email’;
else return

NUMBERFORMAT(fieldname18+fieldname64+fieldname65+fieldname66, ‘GBP’);
})();

https://wordpress.org/plugins/calculated-fields-form/