Qualifying for 250K Capital gains exemption on sale of property

I have a condo in NYC that I lived in from March 2013 until October 2018. In October the property became a rental property.

 

At some point in the next year I would like to sell the property. 

 

I read that if the seller occupied the property for 2 of the last 5 years he qualifies for the 250K capital gains tax exclusion.  

 

I think that means I need to sell it by October of 2021 to qualify for the full 250K cap gain exemption.

 

Oct 2016 - Oct 2017 Owner occupied

OCt 2017 - OCt 2018 -Owner occupied

Oct 2018 - Oct 2019 - tenant occupied

Oct 2019 - OCt 2020 - tenant occupied

Oct 2020 - Oct 2021 - tenant occupied

 

 

After Oct 2021, each month of non-residency in the last 5 years will cost me 1/24 (4.167%) of capital gains exemption.  That's $10,417 in exemption.  At a 15% tax rate that is worth $1562.

 

Am I thinking about this correctly?