In this case, instead to use the “PREC” function, I recommend you to use the following mathematical operation:
ROUND(0.345*100)/100
But the fieldname8 is this
(function(){
if(fieldname4 <= 0) return '0.00';
if(fieldname4 <= 100) return '1.50';
if(fieldname4 <= 500) return '2.60';
if(fieldname4 <= 1000) return '4.85';
if(fieldname4 <= 2500) return '11.00';
})();