I believe you may be conflating two related, but separate issues:
- Maxxing out the 415c limits; and
- Money into the MBCBP


For the 2024 415c limit of $69K, some math applies: If you make (expect to make)--
--$345K+, then you will get the maximum possible company contribution of $58,650 ($345K x 17%), leaving you $10,350 short of the $69K limit (and everything above $345K puts 17% into the MBCBP)
--$270,588.24 - $344,999.99, you will get between $46,000 - $58,650 company contribution (@17%), leaving you $23,000 - $10,350 short of the limit (no unforced MBCBP contributions)
--$78,632.48 - $270,588.23, you will get between $13,367.52 and $58,650 company contribution (@17%), leaving you $55,632.48 - $23,000 short of the limit (no unforced MBCBP contributions)
--Under $78,632.48, you mathematically cannot hit the $69K max, considering the 75% limit on contributions (and MBCBP does not apply)

If you're in the first category, and you don't want "extra" money in the MBCBP, you need to make $10,350 (exactly) worth of elective deferral contributions (pre-tax) to the 401(k) sometime during the year. If you go over the $10,350, you will force the difference into the MBCBP later in the year (so, in this scenario, you'd want to wait at least until you've passed $345K, or the 31 Dec paycheck). You will get 17% of everything above $345K into the MBCBP.

[EDIT: I guess if you really don't want anything in the MBCBP, you could forgo the elective deferrals altogether and avoid any MBCBP contributions up to $405,882.35 of income. I'd be interested to hear any reason that supports this play, though--I can't think of one.]

If you're in the second category, and you don't want "extra" money in the MBCBP, you need to make the exact contribution (between $10,350 & $23K), and unless you have a very accurate way to project your pay (not typical for line pilots), you'll need to time it to get it done on the 31 Dec paycheck.

If you're in the third category, all of the second category applies, AND you'll need to make after-tax contributions to make up the difference (between $23K & ~$55.6K) in after-tax (401a) contributions

This was the situation before the MBCBP. For unknown reasons, we cannot designate a dollar amount, only a percentage amount for our contributions (I submitted a resolution via C44 to fix this; nothing heard since)--so, it requires monitoring carefully if you're in the second or especially the third category. This is what I did in years before the MBCBP. For the 31 Dec paycheck, keep in mind that the "buckets" get filled in the order 401(k) elective deferral; company contribution; 401a after-tax--you can't overfill the total, but you just might put yourself in a situation you don't want if you fill up too fast before 31 Dec, thus limiting your elective deferrals and/or forcing money into the MBCBP that wouldn't otherwise have done so.

NOW consider the MBCBP. It's actually straigtforward, once you've digested all of the above: if you want to avoid the MBCBP, do what was always the case to hit the limit on the 31 Dec paycheck, and you'll have done so. Two caveats: One, real-world vagaries in our pay can make it difficult to avoid over-funding (say) the 401a, which then forces later company contributions to the MBCBP and/or prevents further elective deferrals. Two, once you have the first-world problem of exceeding the compensation limit of $345K (2024), you cannot avoid money into the MBCBP.

Alternatively, if your goal is to get as much money into the MBCBP as possible--i.e., you want to max the $69K and put as much more as possible above that into a tax-deferred retirement vehicle--then simply up your contributions either to the 75% max (100% for profit sharing, minus taxes per the form they send out) or as high as your budget can stand (as small a paycheck as you need) for a given pay period. For current-year taxes, you'll want to ensure you hit the $23K elective deferral max before the overal $69K; if Roth is your goal, same answer into the Roth 401(k); if getting it out of Fidelity is your goal, max the 401a then roll it out (into a Roth IRA would be my first target if I went this route, but I'm a current-year tax guy). This has been my strategy since the MBCBP came into existence: I hit the limit on the 30 Sep paycheck (not going over), then Oct - Dec were 17% to MBCBP. For 2024, I've been maxxing everything in order to get the 17% for the rest of year flowing to the MBCBP.

Obviously, there's also a blended in-between option, where you make some amount less than 75% contributions to hit the limit early (but not "too" early), putting some extra money in the MBCBP, but not all that you theoretically could have.

"Everyone's situation will be different," as the standard disclaimer goes, along with "don't take financial advice from a pilot." Figure out which version works best for you. Good luck!