Help needed with script that will allow calculation to be overwritten

Hello, I do not know a lot about scripting and have tried to do this on my own, but have failed.

I have a form that calculates a deposit based on ContractTotal. If  ContractTotal is less than 3500 it uses the ContractTotal. If it is over 3500 it will caluclate a deposit number by dividing the ContractTotal in half.  Everything works great, but now comes the ask. Some individuals would like to edit the 50% option.  Is that possible?

My script - very simple 

var Dep50 = this.getField("ContractTotal").value;
if( Dep50 < 3500) event.value = Dep50;
else event.value = Dep50 / 2;

Explanation of what is requested: 

ContractTotal = 10000

it will figure the Deposit as 5000

What my guys want is the ability to overwrite the 5000 with something else, like 6000

It should be noted that the form uses the Dep50 value to figure a Contract Balance, which is ContractTotal - Dep50. 

Thank you for help.