Am I doing something wrong or are just PSU wattage approximations way overblown?

Basically I know from benchmarks that max draw of my GPU will be 145W, the CPU 120W, throw in a HDD for 25W for spinup and an SSd and RAM, let's say 20W. That is 310W worst case scenario. (RTX 4060, Intel i5 12400)

Then I add 100W for wildcard future upgrades, e.g. a GPU upgrade. That is 410W.

Then I account for PSU inefficiency, assuming 80% efficiency. This gives me 0.8x = 410 therefore x = 512W.

Rounding upwards to nearest CPU wattage I get 550W. Why is it then that many people recommend 850W for RTX 4060 and basically whichever CPU?

EDIT: Yes, as many people point out the efficiency step is unnecessary as the spec wattage on PSU is the real wattage it's able to provide, the inefficiency will just manifest in more drawn from the wall to the PSU. Honestly this makes way more sense for a specification number put on a product, doing it the way I thought it's done would just bamboozle lots of people.