Note: standard pressure is 29.92126, if you're trying to test for zero.
Like the FAA, this model uses the dry air approximation. Density altitude is also affected by humidity. For a more exact calculator that includes the dewpoint, go here. That calculator will yield the dry-air result if you set the dewpoint at -65°C (-85°F).
On the hottest, most humid day of the year, the dry-air approximation will generally give a result that is too low by 500 feet at most. If this is critical to your operation, it may be time to re-think the whole thing. Your airplane's performance numbers were generated by skilled test pilots with brand-new engines, on smooth runways. What are you flying and where?