401K contributions. when to start using the EMPLOYEE PRE-TAX CATCH-UP

I did some calculations, and I know that if I set my 401K Pre-tax contributions to 22% I will be just under the maximum amount before needing to use catch-up pre-tax contributions. There are fields for both PRE-TAX and EMPLOYEE PRE-TAX CATCH-UP contributions. Are these two fields additive? Does the catch-up election become effective before you are at the normal maximum?