To be fair, how in the world am I supposed to calculate such things and provide a satisfactory number?

The vast majority of my conversations and discourse don't require or benefit from giving a range or specific value to such words. Would the preceding sentence be better if I said 90%, 95%, 99.5% instead of 'vast majority'? And to be honest, most people - especially myself - aren't at all accurate with probability estimates, let alone calculations.