Hi Doug,
If the field is manually set (by entering a value into it) a blue triangle appears in the top right corner of the field. Once it is marked as manually set no subitem changes will roll up into it.
If you don't want any value in it you can always set it to 0 - if you do want it to roll up again you can click on restore when you click on the field to enter values.
Hope this helps,
Robin