Hi Jose,
Do you want the field on the RITM to reflect the sum of the variables as soon as they are updated on the task? If that is the case you can write an After Update Business Rule on the task table and from there access the RITM field and update it with the calculated value.
You can access the task variable using current.variable,field_name in the business rule. Also when you are calculating the sum do not forget to check the type of the input. If the type of the input is an object you can convert it into a string using toString() and then pass it into the parseInt method which would convert the string into a number.
The basic script can look something like this:
var sum = 0;
var tasks = new GlideRecord('sc_task');
tasks.addQuery('request_item',current.request_item);
tasks.query();
while(tasks.next()){
sum = sum + parseInt(tasks.variables.percent.toString());
}
//now display the value of the "sum" variable in whichever field of the requested item you want
