It's not clear to me what this tooltip is talking about:
If the custom fee is set to 1000 satoshis and the transaction is only
250 bytes, then "per kilobyte" only pays 250 satoshis in fee, while
"total at least" pays 1000 satoshis. For transactions bigger than a
kilobyte both pay by kilobyte.

Is "total at least" an old feature which has since been removed?