Your background in property management is incredibly useful -- the experience you're gaining is as important as the capital you're raising.

Is there a way to calculate or do I just need to gain experience doing it?

Assuming this was their personal residence and All cap gain is exempt under section 121, and was never rented (no depreciation recapture.)....1) return of basis-no tax2) cap gain-cap gain tax if not exempt3) interest-taxed as his ordinary income tax rate (no ss/med)

Can I pay capital gain on this $ 500 k or I still have to pay regular income tax on it?

I'm a big believer in direct bookings but you'll need to utilize the existing marketplaces first to gain traction then invest in your own site.

You can sell with a monetized installment sale, which results in 30 year deferral of capital gains tax, and provides you with 93.5% of your net sales proceeds in cash.

The market sometimes (but not always) gives us gains, tenants and time result in paid down mortgages, which results in equity increases.  

When you later sell that property you will not get the full amount of gain tax free as with your other primary residence. 

And I'll turn that into 10-15% gains, or much more...

Reap the benefits of what you have gained so far, most likely almost tax free.