Yep. 3-field approach for selects with an "other" option is a fantastic approach. Just wanted to add an alternative to the IF() statement approach. You can also use COALESCE() for this.
coalesce(${field1},${field2})
Your calculate using this formula will prioritize ${field1} if it contains data (e.g., the 'other' field), or ${field2} is there is no data in ${field1}. Here is a slightly more relevant example: