Greenhouse heater size? calculation vs. table

[tropterrarium]

Now heating: Modine Hot Dawg seems to be solid. I calculated BTU as glass area x delta T/R-value and get ~ 18,000 BTU requirements
2000 sqft x 30 (from 30 to 60F)/3.4 for low E double pane glass.
On greenhouse megastore, HD30 with 24K output BTU is indicated for GH up to 250 sqft, while up to 550 sqft suggests a 48K output BTU. Should I tust my calculations, or manufacturer's indications?

---------- Post added at 04:10 PM ---------- Previous post was at 04:04 PM ----------

Second: Relying upon a single heater is not ideal. But the one that is of appropriate size is already the smallest one.

One option may be to plug electric heaters in swamp-cooler outlets and set them as emergency back-up in the cold period. I have about 16A available for that. A space heater running at 1500W (so around 16A) produces about 5K BTU. May be sufficient to prevent a catastrophe. Also cheap at ~$60.