Button with JavaScript Update

{!REQUIRESCRIPT("/soap/ajax/13.0/connection.js")}
var l = new sforce.SObject("Opportunity");
l.id = "{!Opportunity.Id}";
l.field__c = true;
result = sforce.connection.update([l]);

alert('This may take a minute to calculate.');

//delaying reload to help get the api time
setTimeout("window.location.reload()", 100);