HPSM arithmetic - calculate value (addition) of several fields


I have several custom fields in probsummary - field1, field2, field3, field4, etc.

In those fields there will be manually added numbers - ex: field1=7, field2=5 etc.

Each time a new value is added in any of those fields, there should be an arithmetic operation: addition on another field, lets call it: overal.result

I tried this through FC/Calculations but obivously i can't hit that syntax right... I've added "true" to all required actions, like add/update/display/initial and in the calculation field i'm trying something like overal.result in $file=field1 in $file field2 in $file, etc. but obviously this is not the right approcah.

Can someone assist, please ?