I'm no expert either so I write this to clarify for myself and prompt corrections to my understanding of the issue. I think....

The $270k restriction is a separate calculation from the 403b limit. If you max out your 401k at $18k then $38k is left to reach the $54k 403b limit. At $237,500 the 16% will get you that $38k. So the 403b can be maxed out well before $270k.

The $270k is like a rich person restriction. Congress figured if you make this much you don’t need a tax break on company contributions. For example, if you put zero in your 401k, at $270k the 16% would be $43,200, well below the $54k limit, but the company contributions to PRAP have to stop anyway. You can continue to put 401k money in, even after $270, because this isn’t company money.

In either case, once either limit is reached, the 16% goes to the RHA. It’s allowed because this pretax contribution is not to an individual but to a fund from which the individual cannot take a cash distribution and is not inheritable as part of an individual’s estate.