Mid-month vacate

If a tenant wants to move out mid-month, and they give the proper notification, would you just pro-rate the rent for the last month for the appropriate time period? I’ve got a tenant that wants to move out of the 20th of August, her rent is $600/month so I just:

$600/31 days=$19.35/day
$19.35*20days=$387 due on first of month

Makes sense right?

It depends on state law. Calendar day states like CA require proration. Rental period states allow you to charge for the whole month and there is no requirement to prorate for an early departure. If I were to prorate, then your calculation is what I would use.