Like
@brycejones posted, the 300 ft necessary for the main corridors was public knowledge. Heck, here is an article from several days before the attack.
https://www.cnn.com/2025/06/17/middleeast/iran-fordow-nuclear-site-latam-hnk-intl "Its main halls are an estimated 80 to 90 meters (around 262 to 295 feet) beneath the ground"
Or there is this even earlier article, that puts them up to 360 feet below ground.
https://www.csis.org/analysis/three-things-will-determine-irans-nuclear-future-fordow-just-one-them "Fordow is believed to be buried 80–110 meters underground, near Qom, and was built to withstand airstrikes."
Here is an article from 3.5 years ago:
https://apnews.com/article/europe-m...iran-nuclear-dd40e4a610c2f35aff25353bbcf3a91c "But burying the facility under some 80 meters (260 feet) of dirt and rock while not informing international inspectors as required only heightened Western concerns."
Basically, to answer your question, how do I have better info? For starters, I can read.
Then, according to my Persian coworkers, triple that depth for the deepest corridors. Take their comments with a grain of salt though, since that could very well be internal Iranian TV propaganda.