Assuming you’re in the U.S., **a higher credit limit is useful for purchases you can already afford.** Given that you pay in full and have no upcoming loan application, I’d prioritize consistent, on-time payments and request a limit increase when eligible.

The key is that **utilization is based on the balance reported to the credit bureaus**, typically your statement balance. Your total spending during the month can be higher than that reported balance. Paying before the statement closes can lower reported utilization; paying by the due date keeps your payment on time. [FICO explains the distinction here.](https://www.myfico.com/credit-education/credit-scores/amount-of-debt?sd-noredirect=1)

For your airline ticket, assuming a $1,000 limit and no other charges:

| What happens | Reported utilization |
|---|---:|
| The $400 balance is reported | 40% |
| You pay $350 before the reporting date, leaving $50 | 5% |
| Your limit rises to $10,000 and $400 is reported | 4% |

Ask your issuer when it reports balances; statement closing is the usual timing.

**The “always below 30%” advice is a guideline, not a hard boundary.** Lower reported utilization generally helps your score, including below 30%. A reported 40% balance could lower your score temporarily. Most FICO models use the latest reported balances, so the utilization effect generally improves when a lower balance is reported. Some newer models, including FICO 10T, also consider trends. [FICO’s utilization guidance](https://www.myfico.com/credit-education/blog/accounts-credit-utilization-ratio)

My recommendation for your situation:

- **Buy the ticket if it fits your budget**, and pay the full statement balance by its due date. You do not need to carry debt or pay interest to build credit. [FICO’s explanation](https://www.myfico.com/credit-education/blog/carry-credit-card-balance-myth)
- **Request a limit increase when eligible.** Check whether the request involves a hard inquiry, which can temporarily affect your score. Issuer policies vary. Keep spending within the same budget after an increase. [Capital One’s explanation](https://www.capitalone.com/learn-grow/money-management/credit-limit-increase-affect-credit-score/)
- **Use an early payment when you want lower reported utilization**, especially before applying for credit.

With a ten-year horizon, an occasional utilization-related score dip deserves little attention compared with building a long record of timely payments. A limit increase adds convenience; early payments give you control while your limit is small.