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This assumes people spend at the 33% level on housing.

You'd have to actually know what people are spending on housing, after accounting for spouses/roommates.

It could end more being more or less than 23 million, easily.

e.g. I spend ~22%

Also, some people will own their home or live at a family owned home.

In SV owning your home or living in family owned one for the avg tech employee is unlikely, and 33% is probably less in SV not more

That's only the employee portion, you also spend money on office space.

Also some of the employee's non-rent spending will go to local businesses which will spend some of that money on rent and some of that money on salaries and so on with the new set of employees.

You're applying the "one-third rule" to pre-tax salary, but it's generally applied to take-home pay.