Did you meet the other two, peak and RMS? Do you know what the specs mean?
https://forum.audacityteam.org/t/measure-between-23db-and-18db-rms/32770/16
I posted a process to take you from raw recording to finished ACX-Compliant submission, but if you’re close with what you have, you may only need a gentle push from Noise Reduction. Generally, the less processing, the better.
Drag-Select a portion of Room Tone only. No gasps, no breathing, no mouth ticks, no other sounds. Effect > Noise Reduction > Profile.
Select the whole chapter by clicking just above MUTE. Effect > Noise Reduction: 6, 6, 6 > OK.
Analyze > ACX-Check.
I was clear in the first sentence of the comments that I understand a dividing line between technical conformance and theatrical presentation. Mouth ticks, asthmatic breathing, and harsh SSS sounds will not prevent you from passing the ACX robot, but may very well fail Human Quality Control. Why would anybody pay to listen to vocal presentation that sounds like sandpaper on the ears?
The goal, expressed in different form by ACX, is listening fascinated to somebody telling you a story over cups of tea.
Since your question was about noise floor, I think we’re done, but if you want further analysis and comment, post back.
Koz