Am I estimating capital gains correctly?

[Julie Hill]

Hey all, I have 2 stupid q's:

1) I need to put aside $ from a flip we recently sold for capital gains.  I am calculating 20% from the net profit (we are in CO)... including AFTER holding costs and reimbursing our own $ that we also used to fund the flip.   basically the most bottom of bottom lines after every single expense is taken into account.  Is this generally the way this should be estimated?  And should I do a P&L for the flip to show all of this?  (would be easy to do because I have everything already in a spreadsheet)

2) When do we actually pay this?  When our CPA does our 2024 return next spring?  I know... really really stupid q but I tend to really overthink everything regarding taxes lol.  

TIA!!