It can’t be done – predicting, that is. You need data and real-life conversations and truly, time and experience and risk and all the rest. You can guess, and fantasize of course – but anything remotely reliable just isn’t. No matter how many spreadsheets and models and research you do … ask yourself how much you are willing to risk/invest in finding out. To put your product out there and get actual numbers and feedback. How willing are you to take the answers the market gives you – including “nope”? It’s got to be an amount you are willing and able to actually, literally – lose.
Go through the hard work of making peace with the potential of that loss. If its friends and family money – make sure they understand that and that you are willing to understand it too. No matter how confident you are that your idea will succeed – understanding and emotionally embracing even, this concept will help you be dispassionate about the outcome and the potential turns in the road that you will need to navigate.
Nothing is sacred in entrepreneurship – really. Nothing.