The confusing game of how to max out mega backdoor ROTH to the dollar?

[knightrider]

My megacorop allows mega backdoor ROTH. So each year I max out my 401k by April. Then in June I contribute 50% after tax to my 401k. I want to hit the 2023 $69k limit to the dollar but don't know how.

Here's what I see when I login to my workplace 401k below. If I stop my after tax contributions now will I get close to the $69k limit? I think I also get a "true-up" after the year ends. That kind of messes up these calculations!

[alfaspider]

Does your employer have an excess benefit plan if you go over or does it simply reject excess contributions?

[clipson]

Pre-tax + Roth + any company contributions + after-tax <= 69,000

Catch-up contributions do not count against the 69K limit.

You already know your pre-tax and Roth since you maxed out.

If you get a true-up match, that will count against the 69K even if it happens after the close of the year. You can back into the total match you're entitled to if you know the maximum match you're entitled to. For example, if you put in 6%, your match ends up being 4.5% and if you know your contributions represents more than 6% of your eligible salary, then you know the match is going to 4.5% of your estimated salary for the year.

If you're getting an automatic employer contribution, you should be able to estimate that for the rest of the year.

After-tax = 69,000 - pre-tax - Roth - estimated full-year match (incl. true-up) - estimated other company contributions.

[knightrider]

Does your employer have an excess benefit plan if you go over or does it simply reject excess contributions?

Yes they have an excess plan. I have around $5k in it. I am not quite sure how it works so I would prefer to avoid adding to it..

[SkatingtoFire]

I created my own spreadsheet that tracks this by pay period. Rows are pay period and columns are pay $, % deferral Roth/pretax, Roth/pretax $, % deferral after tax, after tax $, % company match, company match $.

Roth/pretax $ + after tax $ + company match $ < $69k

My megacorp plan does not have a true up match or excess plan. I have to adjust % deferral to ensure getting full company match, while staying under the $69k limit. I am able to get within about $30.00 of the $69k limit, which is only 0.04% error.

When adjusting deferral %, there may be a 1 or 2 pay period delay. I’ve figured out the delay between me making my % deferral election changes and when it gets reflected in my paycheck by trial and error to make the whole thing work.