I find multiple discrepancies and oddities with your calculation that makes me question it. I hope you can help share some light on them.

you are ignoring the other 5 characters on the enemy team in your calculation. in a match as a tank, you don't just want to be able to survive 1 damage type or 1 enemy. You'll divide it by the biggest threat as the game progresses. with 300+ Physical armor and no magical armor and you'll be ignoring all the magical damage in the match. a Casters full combo's will set you down more then half the suggested life total if ignored like that.

For you to reach 306 Phys Armor & 4295 Health on Sevarog, With partial Stacks of Leviathan and Passive, you'll need all 6 items. The items are: Razorback (Crest), Raiment of Renewal, Leviathan (84 extra life), StoneWall, Wardens Faith & Dynamo. https://media.discordapp.net/attachments/1007490150638559295/1058695026332409856/image.png?width=612&height=676 <--- Practice Sevarog, Ignore the spell Haste i got the Maximize haste effect active.
For a total of 15000 Gold. 15k gold will take over 30 minutes to get even with a fairly good kill/Assist count: https://docs.google.com/spreadsheets/d/1rju3wnp3AkxXbvDDpqeLzlYbQ7_bZXxAZIH5KvZJvFM/edit?usp=sharing <--- someone did gold math. It's not perfect and i'm not certain it is correct. but it is damn close based on my match times and gold amount.

As a Tank you'll not get kills and will have a harder time farming CS compared to the Carry. therefore you won't be even on the amount of items. it would be more fair to compare stat numbers regarding 4-5 items vs the carry at 6 items.

Carries regularly get higher attack speed then 2 attacks per sec. 17-18 hits takes 9s. But thats ignoring any and all other abilities Drongo has. Here is a Fairly nuanced build by Joey: https://www.youtube.com/watch?v=GI0eW91KAT8 first shot deals the most dmg.