idea of basic risk

there is an idea of basic risk i was using successfully for some time.
to estimate chances when the probability is unknown
and it is: it might succeed or not and it might fail or not (the inverse of) .combination of both probabilities translates to math of 0.5*(1-0.5) , i guess multiple depended steps might look like 0.5*(1-0.5) * 0.5*(1-0.5) * 0.5*(1-0.5).

it is surprisingly similar to correct numbers. you get probabilities like 0.25 which is 1/0.25 is 1 in 4
where the correct numbers in startups might be like 1 in 5 (80% failure rate).

you can assume risks better than 50:50 for success or failure if you know the numbers if you dont know the odds 0.5 seems ok.