You can write your equation in a new function, and then you would only have to call the function to re-apply the computation.
Open the field calculator, click on the
Function Editor tab,
new file then edit the function. Select
load. Note that
ok will not be available at this point.
Go back to the
Expression tab, select the
custom function group and double click on your function. Select the field to be updated, and hit
If you want to only use field values and constants, you can edit the default function signature, which starts with
value1. In my example I have removed it, but you could use this field(s) to pass new arguments.
You can then access the feature field values using
feature["fieldName"].... be carefull to call this function only on layers containing the said fields!
def funcTestCalc( feature, parent):