New cap rate vs old cap rate

[Julian Riezgo]

So I understand the cap rate tells you the NOI on a property and the ROI you would make if you paid cash for the property, and that cap rate is the NOI ÷ Purchase price....but my question is how do you calculate the new cap rate after rehabbing a property, raising rents, increasing NOI and using the new cap rate to determine whether you'd be able to pull most, if not all of your downpayment back out....

Basically what's the new cap rate of a property after a BRRRR and how do you calculate it....

For example, if a markets cap rate is 8%, but the property I buy is worth 100k with a 10% cap and a NOI of 10k...but after rehab...the property has a NOI of 15k.... would I then divide the new 15k NOI by 10%(old cap of property pre rehab) or 8%(average market cap rate)?

Am I correct in thinking its better to buy a property that needs rehab that has a slightly higher cap rate then the market cap rate? That way I can pull out most of my downpayment? 

I know banks use NOI and DCR for commerical but I think this could still help with BRRRRs on residential property

Any help or advice would be suppperrr helpful