All good questions. My long answer is that there's sadly not really a short way. You need to have an idea of income and expenses, each of which can be trickier than we'd all like. Here are a couple of shortcuts for you:
For a simple down & dirty, no effort, there are a bunch of apps that can assist. Look in your phone.
For a little more effort, you can also design your own Excel spreadsheet, and keep a template copy or three of them in your cloud. (e.g., one for flip, one for hold, one for rehab, one for wholesale, etc.) Put it in (icloud, Gdrive, Dropbox, etc.) and then you can access it from your phone, tablet, etc. whenever you need it. save under an address to preserve your template. My bet is there are a few sheets in the "docs" section here that can get you started.
Wherever you go, you'll have your answers-- and since you worked them out and created them, the results are presented on your own terms with your own biases and preferred analyses already built into the sheet.
This is what I do, and to me, that's much more valuable than "quick."
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