Let's suppose we want to set a custom field called "Urgency" depending on issue "Priority" and the value of another custom field called "Impact". We associate a numerical value to issue "Priority" and to custom field "Impact". These two values may be applied to a mathematical function to calculate another numerical value that will represent the "Urgency" of the issue. In our case we simply have summed both values, but we could have used a much more complex formula. Once we get the result of the sum, we revert the process, associating each possible result of the sum to an options in custom field "Urgency".
Using post-function Set a field as a function of other fields we associate a numerical value to each possible value of issue "Priority", and this value is stored in auxiliary field "Ephemeral number 1".
Using post-function " Set a field as a function of other fields" we associate a numerical value to each possible value of custom field "Impact", and this value is stored in auxiliary field "Ephemeral number 2".
Using post-function " Mathematical and date-time expression calculator" we store in "Ephemeral number 1" the result of summing "Ephemeral number 1" and "Ephemeral number 2".
Note that:
Using post-function Set a field as a function of other fields custom field "Urgency" is set depending on the result of the former sum, which is stored in "Ephemeral number 1".
Once configured "Create Issue" transition looks like this:
Set a field as a function of other fields
Mathematical and date-time expression calculator