Coworker: What makes you so sure?
Me: Math
I had a somewhat similar question answered by me with "I did the math."
Having just switched jobs and finally able to begin 401k contributions again I set the rate to deduct at something like 36%.
The payroll person that goes and sets that up I'm sure did the quick math to know that my salary at that contribution rate would blow past the yearly maximum (if it went through all 26 payment periods of a calendar year). They came up to me with their printout, pointed at the rate I entered, and asked "Did you mean to do this?"
I had started in the middle of the year, my contribution rate at the previous employer was intentionally lower because I it was easy for me to adjust that in between every paycheck (and I was choosing to ramp up contributions throughout the year and prior to known bonus checks), and there was of course the 2 month dead time between ending my previous employment and being able to enroll in the new company's retirement plan.
So, I just said that I had done the math to get myself to the yearly contribution limit. They said "Okay, if you're sure," and gave "The Look".
I sat near them so would overhear them going through some non-sensitive employee on-boarding items if people had questions.
A memorable one was around the topic of what to do with their old 401k. Here is how their conversation went.
PP: = Payroll Person
NEW: = New Employee
NEW: Oh, I already cashed out my old 401k.
PP: Okay, use it for anything fun?
NEW: Yeah, we used it for the down payment of a home improvement loan so we could put in a pool.
PP: That's great!
NEW:Plus, the market is down since then. So, I'm coming out way ahead!
PP: Good for you!
This is when I quietly gave "The Look".