I’ve heard it attributed to the 1.08 constant with an assumed 30°F ΔT across DX coils, but was 400 cfm/ton codified in any ARI/ASHRAE doc or just lab practice from the 1970s? From an efficiency standpoint, we routinely see better latent performance and higher compressor IPLV at 325–350 cfm/ton with ECM blowers and VFD supply fans — curious who first popularized 400 and why it stuck.
If you dig into AHRI 210/240 older editions, they default the indoor test airflow to ‘400 cfm/ton’ when the OEM doesn’t specify, and ACCA Manual D/S examples piggybacked on that. FWIW, a 30°F coil ΔT isn’t typical at that flow — think about 18–20°F — so the ‘1.08’ back‑solve story is more folk math than origin, not carved on a stone tablet. If you’re running 325–350 with ECM/VFD, just verify SHR and coil frost margins in the OEM tables and you’ll usually see the latent/IPLV wins hold.