I posted an answer but realized it wasn't what you wanted. There is some confusion about what you want for Cell3. On the one hand, you say you want the user to enter a vaule in the field, but them you say you want its value to be calculated based on what the user enters and two other field values. It seems to me Cell3 should be the field that the user enters the percentage and the calculated field's (Cell4) script could then be:
// Get first field value, as a number
var v1 = +getField("Cell1").value;
// Get second field value, as a number
var v2 = +getField("Cell2").value;// Get processing field value, as a number
// Get the percentage
var v3 = +getField("Cell3").value;
// Calculate and set this field's value to the result
event.value = (1 + v3 / 100) * (v1 + v2);
You're the man! One logic question: why add the "1" to v3 in the last statement?
---- "event.value = (1 + v3 / 100) * (v1 + v2);" ---
I never would have considered that. Wouldn't v3/100*(v1+v2); work as well?
The latter gives you the percentage of v1 + v2 that you want to then add to v1 + v2. So the former is the same as:
(v1 + v2) + v3/100 * (v1 + v2)