Thanks to @dooglus for asking about this tooltip in Issue 12500.
Reference:  https://www.github.com/bitcoin/bitcoin/issues/12500

I would also appreciate it if someone can confirm that 1 kilobyte in this field indeed represents 1,000 bytes rather than 1,024 bytes (if it's supposed to be 1,024, then I'll gladly make the necessary changes to reflect this).

make check
bitcoin-qt
# Hover over the 'per Kilobyte' note to see the updated tooltip