That’s actually “normal prescription”. In the earliest days of tests and aids, best results were had with a rule: “Provide gain to cover about half the HL”. This was just an approximation, but worked OK. Even with better testing and much improved aids (which can match a target better), the best fittings deviate only slightly from “about half the HL”, guided by decades of experience biasing “half” one way or the other.
Why “half”? First: if you stuck 50dB (a LOT) at 4kHz in your ear, after years of not hearing 4kHz, you would freak out, overwhelmed by all the squeaks in the world. Even 15dB@4kHz really surprised me walking across the wood floor or hearing the dryer squeal.
Second: it is mostly not necessary to boost the world to 0dB or even 15dB HL level. “Threshold” of HL is well below the sounds we really want to hear. 20dB HL is not a significant loss. If you got close to 30dB HL you’d hear far better than you have in years.
Third: most (not all) hearing loss involves “recruitment”, a bad name for the fact that there is loss at threshold but relatively less loss at higher sound levels. I tested my equal-loudness for soft, medium, and loud tones.
When soft I can’t hear highs well. But at “pretty loud” I hear highs pretty good. When I turn up music in my car it does not sound muffled. But when my housemate speaks soft, it is muffled. Simply getting “halfway” from soft to medium would make me hear her better.
And articulation tests show that just a 10dB boost from soft speech to medium in my ear makes me understand very well.
So a “half LH fit” works, for no single reason, but because of a lot of factors that on the whole lead to “half” as a good goal.
That can still be a shock to a disused ear so the audiologist might start from 40% or less, with plan to increase the gain as you learn those high sounds again.
BTW, it is not just “can you hear 4kHz tone?” I have found other tests which show I can no longer differentiate music boosts at 1k 2k or 4kHz, something I used to do for a living (sound-guy). I still know 250 from 500Hz instantly. Damaged then disused ear-nerves forget what things sound like, and the re-learning takes time, months.