Allow user to override javascript calculation

I am a newbie to javascript, but I managed to make a formula that works. However, the problem I now have is that I need to tweak it so that the calculation does NOT happen if the user of the form manually overrides the data populated by the calculation. How can I tweak it to do that? This is the current formula:

getField("EL_Effective").value = getField("UMB_Effective").valueAsString;