Median test is for abuse. It is possible (probably not likely) to be rejected under median income.
I would file ASAP if you know you are going to file.
Every trustee is different.
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For chapter 7 you will need your expenses to exceed your income. You also need to be under the median income. Essentially they are two separate tests that need to be passed (I & J and the Means Test). If you're below the median income, you've passed the means test and can file for Chapter 7 bankruptcy. It's as easy as that.
Schedules I & J use your actual (and forward looking) income & expenses and may show a positive disposable monthly income (DMI). If your actual net DMI is high enough, in other words, you have sufficient monies left over each month to fund a Ch13, then you could very well be ineligible for Ch7. The choices then would be to have your case dismissed or to convert to a Ch13.
What you do to stay under the means is generally irrelevant. But as @Shooting-For-800 said, the trustee could inquire. But there is nothing in bankruptcy law or the Constitution of the United States says you can't make strategic moves to file. Any way you lower your calculated 6 month average to less than the median income is acceptable. You can change jobs, go part time, take time off, etc.